Liquid crystal display with method for OCB splay-bend transition

ABSTRACT

Before starting the regular display in a liquid crystal display device of the bend alignment type, it is necessary to transition all the pixel regions in the entire display portion uniformly from splay alignment into bend alignment. However, conventionally, when applying a simple ac voltage, the transition sometimes does not take place, and when it does take place, the transition time is very long, and display defects due to alignment defects tend to occur. 
     In the method for driving a liquid crystal display device with OCB cells according to the present invention, a step of applying between an electrode  22  and a pixel electrode  23  an ac voltage superimposed with a bias voltage, and a step of applying zero voltage or a low voltage to the substrates are repeated in alternation preceding the begin of the regular display operation and the regular display operation is carried out after all pixels have transitioned into bend alignment.

TECHNICAL FIELD

The present invention relates to an OCB-mode liquid crystal displaydevice with fast response and broad viewing angle for displaying TVimages, personal computer or multimedia images, a manufacturing methodfor the same, and a driving method for a liquid crystal display device.

BACKGROUND ART

Conventional liquid crystal display devices employ, as one example ofliquid crystal display modes, twisted nematic (TN) mode liquid crystaldisplay elements using a nematic liquid crystal with positive dielectricanisotropy, but these have the shortcomings of a slow response andnarrow viewing angles. There are also display modes with slow responseand broad viewing angles, using a ferroelectric liquid crystal (FLC) oranti-ferroelectric liquid crystal, but these have shortcomings withregard to burn-in, shock resistance, and temperature dependence. Thereis also the in-plane switching (IPS) mode which has extremely broadviewing angles, in which the liquid crystal molecules are driven withinthe display plane by a transversal electric field, but the responsetimes are slow, and numerical aperture and luminance are low. Whentrying to display full-color moving images on large screens, a liquidcrystal mode with broad viewing angle, high luminance and fast displayproperties is necessary, but at present, a liquid crystal display modethat perfectly satisfies all these requirements in practice does notexist.

Among the conventional liquid crystal display devices that aimed for atleast a broad viewing angle and high luminance are liquid crystaldisplay devices in which TN mode liquid crystal regions are partitionedinto two domains to widen the viewing angle vertically (see SID 92DIGEST p.798-801). That is to say, using a nematic liquid crystal withpositive dielectric anisotropy in the display pixels of the liquidcrystal display device, two TN mode liquid crystal regions withdifferent alignment orientation of the liquid crystal molecules areformed, and the viewing angle is enlarged by this TN-mode with twoalignment domains.

FIG. 48 is a diagram showing the configuration of such a conventionalliquid crystal display device. In FIG. 48, numerals 701 and 702 denoteglass substrates, numerals 703 and 704 denote electrodes, and numerals705, 705′, 706, and 706′ denote alignment films. In the alignment regionA, the nematic liquid crystal molecules 707 and 707′ with positivedielectric anisotropy are slightly tilted away from the upper and lowerboundaries to the opposing substrates, forming a larger and a smallerpretilt angle, whereas in the other alignment region B, the size of thepretilt angles with respect to the upper and lower boundaries of theopposing substrates is opposite to that in the alignment region A. Boththe larger and the smaller pretilt angles are several degrees each, andare set to different angles. An example of a conventional manufacturingmethod for forming alignment regions with different pretilt angles atthe upper and lower substrates is spreading photoresist on an alignmentfilm, masking the photoresist photolithographically, and rubbing thedesired alignment film surface in a predetermined direction, andrepeating this procedure a certain number of times. As shown in FIG. 1,with this configuration, the liquid crystal molecules in the centralportions of the liquid crystal layer in the alignment regions A and Bare provided with opposite orientations, and since the liquid crystalmolecules of the alignment regions rise in different directions when avoltage is applied, the refractive index anisotropy with respect toincoming light evens out for each pixel, and the viewing angle can beenlarged. With this conventional TN-mode with two alignment domains, theviewing angle can be made wider than with regular TN-mode, and thevertical viewing angle becomes about ±35° at a contrast of 10.

However, the response time is substantially the same as in TN-mode,namely about 50 ms. Thus, in this conventional TN-mode with twoalignment domains, viewing angle and response are insufficient.

As for liquid crystal display modes utilizing the so-called homeotropicalignment mode, in which the liquid crystal molecules are alignedapproximately vertically at the boundaries to the alignment films, thereare liquid crystal display devices with broad viewing angle and fastresponse that are provided with film phase-difference plates andsubjected to alignment partitioning, but again the response time betweenblack and white display is about 25 ms, and in particular the responsetime for gray scales is slow at 50-80 ms, which is longer than the 1/30s that are held to be the visual speed of the human eye, so that movingimages appear blurred.

On the other hand, a bend alignment type liquid crystal display device(OCB-mode liquid crystal display device) has been proposed, whichutilizes changes of the refractive index due to changes in the anglewith which the liquid crystal molecules rise when the liquid crystalmolecules between the substrates are in bend alignment. The speed withwhich the orientation of bend aligned liquid crystal molecules changesin the ON state and the OFF state is much faster than the speed oforientation changes between ON and OFF states in TN liquid crystaldisplay devices, so that a liquid crystal display device with fastresponse time can be obtained. Moreover, in this bend alignment typeliquid crystal display device, optical phase differences can becompensated automatically, because all the liquid crystal molecules arebend aligned between the upper and lower substrates, and the liquidcrystal display device has potential as a liquid crystal display devicewith low voltage and broad viewing angle, because phase differences arecompensated by the film phase difference plates.

Incidentally, these liquid crystal display devices are manufactured suchthat the liquid crystal molecules between the substrates are in splayalignment when no voltage is applied. In order to change the refractiveindex using bend alignment, the entire display portion has to betransitioned uniformly from splay alignment to bend alignment before useof the liquid crystal display device. When applying a voltage betweenthe opposing display electrodes, the transition seeds for the transitionfrom splay alignment to bend alignment do not appear in uniformdistribution, but around the distributed spacers, at alignmentirregularities at the boundary to the alignment films, or at damagedportions. Furthermore, the transition seeds do not necessarily appearalways at the same locations, which may easily lead to display defects,in which the transition sometimes takes place and sometimes does nottake place. Consequently, it is very important that at least all pixelportions of the entire display portion are transitioned uniformly fromsplay alignment to bend alignment before use.

However, conventionally, when applying a simple ac voltage, thetransition sometimes does not take place, and when it does take place,the transition time is very long.

DISCLOSURE OF THE INVENTION

It is an object of the present invention bend alignment type liquidcrystal display device with fast response, suitable for display ofmoving images and broad viewing angle, in which the transition into bendalignment takes place reliably, and the transition is concluded in shorttime so that there are no display defects, as well as a manufacturingmethod for such a liquid crystal display device, and a driving methodfor a liquid crystal display device.

To achieve this object, according to a first aspect of the invention, amethod for driving a liquid crystal display device, for an alignmenttransition from splay alignment to bend alignment in a liquid crystaldisplay device which includes a pair of substrates and a liquid crystallayer disposed between the substrates; wherein, when no voltage isapplied, pretilt angles of the liquid crystal at an upper and at a lowerboundary of the liquid crystal layer have opposite signs, and the liquidcrystal layer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage to the substrates; and wherein theliquid crystal display driving is performed in the bend alignmentattained by this initialization;

includes applying to the substrates an ac voltage superimposed with abias voltage to cause transition of the liquid crystal layer into bendalignment.

With this method, an ac voltage superimposed with a bias voltage isapplied between the substrates, which makes the transition time shorterthan when only an ac voltage is applied. The reason for this is that,superimposing a bias voltage has the effect that the alignment of theliquid crystal molecules in the liquid crystal layer is disturbed by thebias voltage, and the liquid crystal molecules lean toward one of thesubstrates. Thus, transition seeds appear within a short time andreliably in the liquid crystal layer, and the transition time isshortened. In addition, the transition time be made even shorter byincreasing the effective voltage.

According to a second aspect of the invention, a driving method for analignment transition from splay alignment to bend alignment in a liquidcrystal display device which includes a pair of substrates and a liquidcrystal layer disposed between the substrates; wherein, when no voltageis applied, pretilt angles of the liquid crystal at an upper and at alower boundary of the liquid crystal layer have opposite signs, and theliquid crystal layer is in splay alignment, having been subjected to aparallel alignment process; wherein, before liquid crystal displaydriving, an initialization process is performed, in which the alignmentof the liquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage to the substrates; and wherein theliquid crystal display driving is performed in the bend alignmentattained by this initialization;

includes a step of applying to the substrates an ac voltage superimposedwith a bias voltage, and a step of putting the substrates into anelectrically released state, repeated in alternation so as to causetransition of the liquid crystal layer into bend alignment.

This configuration includes providing a period of an electricallyreleased state after the application of the ac voltage, which has theeffect that the alignment of the liquid crystal molecules in the liquidcrystal layer is disturbed, and the liquid crystal molecules lean towardone of the substrates. Thus, transition seeds appear within a short timeand reliably in the liquid crystal layer, and the transition time isshortened.

According to a third aspect of the invention, a driving method for analignment transition from splay alignment to bend alignment in a liquidcrystal display device which includes a pair of substrates and a liquidcrystal layer disposed between the substrates; wherein, when no voltageis applied, pretilt angles of the liquid crystal at an upper and at alower boundary of the liquid crystal layer have opposite signs, and theliquid crystal layer is in splay alignment, having been subjected to aparallel alignment process; wherein, before liquid crystal displaydriving, an initialization process is performed, in which the alignmentof the liquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage to the substrates; and wherein theliquid crystal display driving is performed in the bend alignmentattained by this initialization;

includes a step of applying to the substrates an ac voltage superimposedwith a bias voltage, and a step of applying zero voltage or a lowvoltage to the substrates, repeated in alternation so as to causetransition of the liquid crystal layer into bend alignment.

This configuration includes a zero voltage or a low voltage applicationperiod after the application of the ac voltage, so that the effect ofdisturbing the alignment of the liquid crystal molecules in the liquidcrystal layer is larger than in the second aspect of the presentinvention. Consequently, the effect that the liquid crystal moleculeslean toward one of the substrates occurs in very little time. Thus, andthe transition time becomes even shorter.

According to a fourth aspect of the invention, in a method for driving aliquid crystal display device in accordance with the third aspect, theac voltage superimposed with the bias voltage is replaced with a dcvoltage.

With this configuration, also when a dc voltage is applied instead ofthe ac voltage, there are periods in which zero voltage of a low voltageare applied after the application of this dc voltage, which causedisturbances of the liquid crystal alignment in the liquid crystallayer. Thus, the transition time can be shortened with this drivingmethod, too.

According to a fifth aspect of the invention, in a method for driving aliquid crystal display device in accordance with the second aspect, thefrequency of the voltage repeated in alternation is in the range of 0.1Hz to 100 Hz, and the duty ratio of the voltage repeated in alternationis in the range of at least 1:1 to 1000:1.

Here, “voltage repeated in alternation” is the voltage when therepetition of the ac voltage application period and the period of theelectrically released that as a whole is taken as one voltage pattern.The following are the reasons for the limitation of the frequency andthe duty of the voltage repeated in alternation.

When the frequency is smaller than 0.1 Hz, then there is almost noalternating repetition, so that the tilting of the liquid crystalmolecule alignment caused by this alternating repetition stops. On theother hand, when the frequency is larger than 100 Hz, then the rate ofthe alternating repetition is too high and approximates an ac voltage,so that the tilting of the liquid crystal molecule alignment caused bythis alternating repetition stops.

When the duty ratio of the repeatedly applied voltage is smaller than1:1 (for example, 1:5), then the voltage applied to the liquid crystallayer is not sufficient. When the duty ratio is larger than 1000:1, thenthere is almost no alternated repetition, and the voltage is almost a dcvoltage, so that the tilting of the liquid crystal molecule alignmentcaused by this alternating repetition stops.

According to a sixth aspect of the invention, in a method for driving aliquid crystal display device in accordance with the third aspect, thefrequency of the voltage repeated in alternation is in the range of 0.1Hz to 100 Hz, and the duty ratio of the voltage repeated in alternationis in the range of at least 1:1 to 1000:1.

The reasons for these limitations of the frequency and the duty ratio ofthe voltage repeated in alternation are the same as in the sixth aspectof the present invention.

According to a seventh aspect of the invention, in a method for drivinga liquid crystal display device in accordance with the first aspect, theliquid crystal display device is an active matrix liquid crystal displaydevice, and wherein the ac voltage is applied between a pixel electrodeof the active matrix liquid crystal display device that is coupled to aswitching element formed on one of the substrates and a common electrodeformed on the other substrate.

With this configuration, the transition time can be shortened in anactive matrix liquid crystal display device.

According to an eighth aspect of the invention, in a method for drivinga liquid crystal display device in accordance with the third aspect, theliquid crystal display device is an active matrix liquid crystal displaydevice, and wherein the ac voltage is applied between a pixel electrodeof the active matrix liquid crystal display device that is coupled to aswitching element formed on one of the substrates and a common electrodeformed on the other substrate.

With this configuration, the transition time can be shortened in anactive matrix liquid crystal display device.

According to a ninth aspect of the invention, in a method for driving aliquid crystal display device in accordance with the eighth aspect, theac voltage is applied to the common electrode.

With this configuration, the transition time can be shortened.

According to a tenth aspect of the invention, in a method for driving aliquid crystal display device in accordance with the fourth aspect, theliquid crystal display device is an active matrix liquid crystal displaydevice, and wherein the dc voltage is applied between a pixel electrodeof the active matrix liquid crystal display device that is coupled to aswitching element formed on one of the substrates and a common electrodeformed on the other substrate.

With this configuration, the transition time can be shortened in anactive matrix liquid crystal display device.

According to an eleventh aspect of the invention, in a method fordriving a liquid crystal display device in accordance with the tenthaspect, the dc voltage is applied to the common electrode.

With this configuration, the transition time can be shortened.

According to a twelfth aspect of the invention, in a method for drivinga liquid crystal display device in accordance with the first aspect, thevalue of the ac voltage is set to a critical voltage that is a minimumvoltage necessary for transitioning the liquid crystal layer from splayalignment to bend alignment.

With this configuration, it is possible to reduce the voltage.

According to a thirteenth aspect of the invention, in a method fordriving a liquid crystal display device in accordance with the fourthaspect, the value of the ac voltage is set to a critical voltage that isa minimum voltage necessary for transitioning the liquid crystal layerfrom splay alignment to bend alignment.

With this configuration, it is possible to reduce the voltage.

According to a fourteenth aspect of the invention, in a method fordriving a liquid crystal display device in accordance with the thirdaspect, the voltage is an alternated voltage averaging over time.

With this configuration, deterioration of the liquid crystal can beprevented.

According to a fifteenth aspect of the invention, a liquid crystaldisplay device including a pair of substrates and a liquid crystal layerdisposed between the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage to the substrates; wherein theliquid crystal display driving is performed in the bend alignmentattained by the initialization;

including a voltage application means for applying to the substrates anac voltage or a dc voltage superimposed with a bias voltage, so as totransition the liquid crystal layer from splay alignment to bendalignment.

With this configuration, a liquid crystal display device with shorttransition time is accomplished.

According to a sixteenth aspect of the invention, in a liquid crystaldisplay device as in the fifteenth aspect, the value of the ac voltageor dc voltage is set to a critical voltage that is a minimum voltagenecessary for transitioning the liquid crystal layer from splayalignment to bend alignment.

With this configuration, a liquid crystal display device with shorttransition time is accomplished.

According to a seventeenth aspect of the invention, an active matrixliquid crystal display device including an array substrate provided witha pixel electrode; an opposing substrate provided with a commonelectrode; and a liquid crystal layer arranged between the arraysubstrate and the opposing substrate; wherein pretilt angles of theliquid crystal at an upper and at a lower boundary of liquid crystallayer have opposite signs, and in a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process, theliquid crystal is in splay alignment when no voltage is applied;wherein, before liquid crystal display driving, an initializationprocess is performed, in which the alignment of the liquid crystal layeris transitioned from splay alignment to bend alignment by application ofa voltage; wherein the liquid crystal display driving is performed inthe bend alignment attained by the initialization; including:

a liquid crystal cell including at least a first liquid crystal cellregion, wherein a liquid crystal pretilt angle at an alignment filmformed on an inner side of the array substrate is a first pretilt angle,and wherein a liquid crystal pretilt angle at an alignment film formedon an inner side of the opposing substrate is a second pretilt anglelarger than the first pretilt angle; and a second liquid crystal cellregion arranged next to the first liquid crystal cell region within thesame pixel; wherein a liquid crystal pretilt angle at an alignment filmformed on an inner side of the array substrate is a third pretilt angle,and wherein a liquid crystal pretilt angle at an alignment film formedon an inner side of the opposing substrate is a fourth pretilt anglelarger than the third pretilt angle, the alignment films having beensubjected to an alignment process directed from the first liquid crystalcell region to the second liquid crystal cell region;

a first voltage application means for applying a first voltage betweenthe pixel electrode and the common electrode so as to form adisclination line at a border between the first liquid crystal cellregion and the second liquid crystal cell region; and

a second voltage application means for creating transition seeds at thedisclination line by applying a second voltage larger than the firstvoltage between the pixel electrode and the common electrode, andcausing transition from splay alignment to bend alignment.

With this configuration, applying a first voltage between the pixelelectrode and the common electrode forms a disclination line between thefirst liquid crystal cell region and the second liquid crystal cellregion, where the bending energy is higher than around it, and applyinga second voltage larger than the first voltage between the pixelelectrode and the common electrode, directs even more energy to thisdisclination line, causing transition from splay alignment to bendalignment at the disclination line.

Consequently, in a liquid crystal display device with thisconfiguration, the splay—bend alignment transition occurs reliably at acertain location (namely at the disclination lines) within the pixelregions provided with many liquid crystal cells, a reliable and fastalignment transition can be ensured, and a high-quality and inexpensiveliquid crystal display device without display defects can be realized.

According to an eighteenth aspect of the invention, in a liquid crystaldisplay device as in the seventeenth aspect, the first and the fourthpretilt angles are at most 3°, and the second and third pretilt anglesare at least 4°.

With this configuration, the ratio between the second and the fourthpretilt angle, and the ratio between the first and the fourth pretiltangle can be large, so that disclination lines with a bending energythat is even higher than the bending energy around them can be formed,and the transition time from splay alignment to bend alignment can bemade even shorter.

According to a nineteenth aspect of the invention, in a liquid crystaldisplay device as in the seventeenth aspect, the direction in which thealignment films are subjected to the alignment process is perpendicularto signal electrode lines or gate electrode lines arranged along thepixel electrode.

With this configuration, a transversal electric field is applied fromthe transversal electric field application portions in a direction thatis substantially perpendicular to the alignment of the liquid crystalmolecules in the liquid crystal layer, so that this transversal electricfield exerts a twisting force on the liquid crystal molecules, andconsequently, transition seeds appear at the disclination line, and aquick alignment transition from splay alignment to bend alignment can beachieved.

According to a twentieth aspect of the invention, in a liquid crystaldisplay device as in the seventeenth aspect, the direction in which thealignment films are subjected to the alignment process is slightly askewto a direction perpendicular to signal electrode lines or gate electrodelines arranged along the pixel electrode.

Making the direction in which the alignment films are subjected to thealignment process is slightly askew to a direction perpendicular tosignal electrode lines or gate electrode lines arranged along the pixelelectrode, a slightly askew transversal electric field is applied to thedisclination lines from the signal electrode lines or gate electrodelines, so that the twisting force on the splay aligned liquid crystalmolecules is increased, thereby assisting the transition to bendalignment.

According to a twenty-first aspect of the invention, in a liquid crystaldisplay device as in the seventeenth aspect, the second voltage ispulse-shaped with a frequency in the range of 0.1 Hz to 100 Hz, and aduty ratio in the range of at least 1:1 to 1000:1.

Applying such a pulse-shaped second voltage and alternating voltageapplication periods and periods in which no voltage is applied, theliquid crystal molecules are disturbed and transition more readily, sothat the splay aligned liquid crystal molecules transition into bendalignment. Frequency and duty ratio are limited to the above ranges toenlarge the transition regions of transition from splay alignment tobend alignment.

According to a twenty-second aspect of the invention, in a liquidcrystal display device as in the seventeenth aspect, the gate electrodelines are in an ON state for at least most of said transition period.

The regions of the disclination lines have a bending energy that ishigher than in the regions around them, and in this situation, thetransversal electric field is applied to the disclination lines from thegate electrode lines, which are arranged transversally with respect tothe pixel electrodes, so that even more energy is directed to them, andthe transition from splay alignment to bend alignment is accelerated.

According to a twenty-third aspect of the invention, a liquid crystaldisplay device as in the seventeenth aspect further includes a liquidcrystal cell that has been alignment partitioned by irradiating UV lighton a portion of at least one of the alignment films formed on the innersides of the pixel electrode and the common electrode so that thepretilt angle of the liquid crystal at that alignment film is changed.

Irradiating UV light on a portion of the alignment films, it is possibleto modify the surface of the irradiated region of the alignment films,and to decrease the pretilt angle of the liquid crystal in the modifiedalignment films. The reasons why the pretilt angle in the alignmentfilms are decreased by irradiation with UV light are not entirely clearat present, but it seems that the UV light breaks up side chains in thealignment surface. Thus, liquid crystal cells with alignment partitionscan be formed by irradiation with UV light.

According to a twenty-fourth aspect of the invention, a liquid crystaldisplay device as in the seventeenth aspect further includes a liquidcrystal cell that has been alignment partitioned by irradiating aportion of the pixel electrode and a portion of the common electrodewith UV light under an ozone atmosphere to flatten at least one of theportions of the pixel electrode and the common electrode has beenflattened, and applying and baking an alignment film on the pixelelectrode and the common electrode, so as to change the pretilt angle ofthe liquid crystal at the alignment film.

Irradiating a portion of the pixel electrode and a portion of the commonelectrode with UV light under an ozone atmosphere, the surfaces of thepixel electrode and the common electrode can be flattened, andconsequently, liquid crystal cells with alignment partitions and varyingpretilt angles of the liquid crystal at the alignment films can beformed by spreading the alignment films on the pixel electrode and thecommon electrode.

According to a twenty-fifth aspect of the invention, a method formanufacturing an active matrix liquid crystal display device includingan array substrate provided with a pixel electrode; an opposingsubstrate provided with a common electrode; and a liquid crystal layerarranged between the array substrate and the opposing substrate; whereinpretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and in a liquid crystalcell in splay alignment, which has been subjected to a parallelalignment process, the liquid crystal is in splay alignment when novoltage is applied; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by theinitialization; includes:

a preparation step of preparing a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process,wherein the pretilt angles of the liquid crystal at the upper and lowerboundaries of the liquid crystal layer arranged between the arraysubstrate provided with the pixel electrode and the opposing substrateprovided with the common electrode have opposite signs;

a disclination line forming step of applying a first voltage for forminga disclination line between the pixel electrode and the commonelectrode, and forming a disclination line at a boundary between a firstliquid crystal cell region and a second liquid crystal cell region; and

an alignment transition step for transition from splay alignment to bendalignment of applying a second voltage larger than the first voltagebetween the pixel electrode and the common electrode, and creatingtransition seeds at the disclination line at the boundary between thefirst liquid crystal cell region and the second liquid crystal cellregion.

With this method, the splay—bend alignment transition occurs reliably ata certain location (namely at the disclination lines) within the pixelregions provided with many liquid crystal cells in the liquid crystaldisplay device, and transition seeds appear reliably, because thebending energy at the disclination lines is higher than around them.Consequently, a reliable and fast alignment transition can be ensured,and a high-quality and inexpensive liquid crystal display device withoutdisplay defects can be obtained.

According to a twenty-sixth aspect of the invention, in a method formanufacturing a liquid crystal display device as in the twenty-fifthaspect, the preparation step includes an alignment process step ofarranging the liquid crystal molecules in one pixel region in b-splayalignment by subjecting them to an alignment process such that a pretiltangle of the liquid crystal on the pixel electrode side becomes smallerthan a pretilt angle of the liquid crystal on the common electrode side,and arranging the liquid crystal molecules in another pixel region int-splay alignment by subjecting them to an alignment process such that apretilt angle of the liquid crystal on the pixel electrode side becomeslarger than a pretilt angle of the liquid crystal on the commonelectrode side.

With this method, b-splay alignment regions and t-splay alignmentregions are formed in the pixels, and disclination lines are formedclearly at the border between them. As mentioned above, the bendingenergy at these disclination lines is larger than around them, so thattransition seeds appear reliably, and consequently, a reliable and fastalignment transition can be ensured.

According to a twenty-seventh aspect of the invention, in a method formanufacturing a liquid crystal display device as in the twenty-sixthaspect, the alignment process step includes alignment partitioning byirradiating UV light on a portion of the alignment film formed on aninner surface side of at least one electrode of the pixel electrode andthe common electrode to change the pretilt angle of the liquid crystal.

Irradiating UV light on a portion of the alignment films, it is possibleto modify the surface of regions the alignment films irradiated with UVlight, and to decrease the pretilt angle of the liquid crystal in themodified alignment films.

According to a twenty-seventh aspect of the invention, in a method formanufacturing a liquid crystal display device as in the twenty-sixthaspect, the alignment process step includes alignment partitioning byirradiating a region of at least one electrode of the pixel electrodeand the common electrode with UV light under an ozone atmosphere,flattening a portion of the pixel electrode and the common electrode,and then applying and baking an alignment film on the pixel electrodeand the common electrode to change the pretilt angle of the liquidcrystal at the alignment film.

With this method, a portion of either the pixel electrode or the commonelectrode or both can be flattened, and consequently, a liquid crystaldisplay device having liquid crystal cells with alignment partitions andvarying pretilt angles of the liquid crystal at the alignment films canbe formed by spreading the alignment films on the pixel electrode andthe common electrode.

According to a twenty-ninth aspect of the invention, an active matrixliquid crystal display device includes an array substrate provided witha pixel electrode; an opposing substrate provided with a commonelectrode; and a liquid crystal layer arranged between the arraysubstrate and the opposing substrate; wherein pretilt angles of theliquid crystal at an upper and at a lower boundary of liquid crystallayer have opposite signs, and in a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process, theliquid crystal is in splay alignment when no voltage is applied;wherein, before liquid crystal display driving, an initializationprocess is performed, in which the alignment of the liquid crystal layeris transitioned from splay alignment to bend alignment by application ofa voltage; wherein the liquid crystal display driving is performed inthe bend alignment attained by the initialization; and

wherein each pixel has at least one transition-inducing transversalfield application portion due to which a transversal electric field isgenerated, and applying a continuous or intermittent voltage to thepixel electrode and the common electrode, transition seeds are createdin each pixel, and the pixels transition from splay arrangement to bendarrangement.

The following effects can be attained with this configuration.

A voltage that is sufficiently larger than the transition voltage isapplied between the pixel electrode and the common electrode, and atleast one transition-inducing transversal electric application fieldportion provided in each pixel applies a transversal electric field,whereby the transversal electric application field portion becomes thestarting point for the transition of the liquid crystal layer in thepixel from splay alignment to bend alignment (that is, it can be ensuredthat transition seeds appear in the liquid crystal layer near thetransversal electric field application portions). Thus, the transitionfrom splay alignment to bend alignment can be carried out fast.

According to a thirtieth aspect of the invention, in a liquid crystaldisplay device as in the twenty-ninth aspect, the transversal electricfield generated by the transversal electric field application portionsis substantially perpendicular to the direction of the alignmentprocess.

With this embodiment, the transversal electric field is applied by thetransversal electric field application portions in a direction that issubstantially perpendicular to the direction of the alignment of theliquid crystal molecules in the liquid crystal layer, so that thistransversal electric field exerts a twisting force on the liquid crystalmolecules, and consequently, transition seeds appear, and a quicktransition from splay alignment to bend alignment can be achieved.

According to a thirty-first aspect of the invention, in a liquid crystaldisplay device as in the twenty-ninth aspect, the transversal electricfield application portions are electrode deformation portions, in whichsides of the pixel electrodes are deformed to protrusions and recessesin a plane parallel to the substrate plane.

The following effects can be attained with this configuration.

The electric field concentrates between the transversal electric fieldapplication portions, which are electrode deformation portions, in whichsides of the pixel electrodes are deformed to protrusions and recessesin a plane parallel to the substrate plane, and signal electrode linesor gate electrode lines arranged beneath the transversal electric fieldapplication portions. Consequently, the transversal electric fieldgenerated like this is stronger than the transversal electric fieldgenerated between pixel electrodes without such transversal electricfield application portions and the signal electrode lines or gateelectrode lines. Consequently, with the transversal electric fieldgenerated due to the transversal electric field application portions,the appearance of seeds in the liquid crystal layer can be ensured, anda quick transition from splay alignment to bend alignment can beachieved.

According to a thirty-second aspect of the invention, in a liquidcrystal display device as in the twenty-ninth aspect, the transversalelectric field application portions are electrode line deformationportions, in which signal electrode lines or gate electrode lines aredeformed to protrusions and recesses in a plane parallel to thesubstrate plane.

The following effect can be attained with this configuration.

The same effect as in the thirty-first aspect of the present inventionis attained due to electrode line deformation portions at either one orboth types of electrode lines.

According to a thirty-third aspect of the invention, in a liquid crystaldisplay device as in the twenty-ninth aspect, the transversal electricfield application portions are deformations in the electrodes and theelectrode lines, in which sides of the pixel electrodes are deformed toprotrusions and recesses in a plane parallel to the substrate plane, andin correspondence to these protrusions and recesses, signal electrodelines or gate electrode lines are deformed to protrusions and recessesin a plane parallel to the substrate plane.

The following effects can be attained with this configuration.

The same effect as in the thirty-first aspect of the present inventionis attained with the transversal electric field application portions,which are deformations in the electrodes and the electrode lines, inwhich at least one side of the pixel electrodes is deformed toprotrusions and recesses in a plane parallel to the substrate plane, andin correspondence to these protrusions and recesses, signal electrodelines or gate electrode lines or both are deformed to protrusions andrecesses.

According to a thirty-fourth aspect of the invention, in a liquidcrystal display device as in the twenty-ninth aspect, the transversalelectric field application portions are transversal electric fieldapplication line deformation portions in transversal electric fieldapplication lines that are deformed to protrusions and recesses in aplane parallel to the substrate plane, wherein the transversal electricfield application lines are arranged in a layer above or below at leastone of signal electrode lines or gate electrode lines and in the samedirection as these, separated from them by an insulting film, andwherein the transversal electric field application lines are connectedto a driving circuit, to which also the signal electrode lines or gateelectrode lines are connected.

With this configuration, the transversal electric field applicationlines are dedicated lines for transversal electric field application,and are arranged in a layer above or below at least one of signalelectrode lines or gate electrode lines, separated from them by aninsulting film, which leads to flexibility with regard to the shape ofthe protrusions and recesses, which can be formed for examplecontinuously along the sides of the transversal electric fieldapplication lines. Furthermore, since the transversal electric fieldapplication lines overlap with the signal electrode lines or the gateelectrode lines, there is little light absorption, and consequently theaperture ratio of the pixels does not decrease. Thus, a redundant designwith a greater degree of freedom.

According to a thirty-fifth aspect of the invention, in a liquid crystaldisplay device as in the thirty-fourth aspect, the transversal electricfield application lines are disconnected from the driving circuit duringregular liquid display after alignment transition.

With this configuration, the transversal electric field applicationlines are disconnected from the driving circuit during regular liquiddisplay after alignment transition, so that no electric field isgenerated between the transversal electric field application portionsformed in the transversal electric field application lines and the pixelelectrodes. Consequently, disturbances in the alignment of the liquidcrystal do not occur during regular liquid crystal display, so that aliquid crystal display device with superior liquid crystal displayquality can be obtained.

According to a thirty-sixth aspect of the invention, an active matrixliquid crystal display device including an array substrate; an opposingsubstrate; and a liquid crystal layer arranged between the arraysubstrate and the opposing substrate; wherein pretilt angles of theliquid crystal at an upper and at a lower boundary of liquid crystallayer have opposite signs, and in a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process, theliquid crystal is in splay alignment when no voltage is applied;wherein, before liquid crystal display driving, an initializationprocess for a transition from splay alignment to bend alignment isperformed by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by theinitialization;

includes at least one of a pixel electrode and a common electrode,wherein a defect portion for application of a transition-inducingtransversal electric field is formed at least at one location in eachpixel.

The following effects can be attained with this configuration.

Having at least one of a pixel electrode and a common electrode in whicha defect portion for application of a transition-inducing transversalelectric field is formed at least at one location for each pixel unit, abending of the electric field (that is, an oblique electric field) isgenerated at the edge of this defect portion. Consequently, this obliqueelectric field exerts a twisting force on the liquid crystal molecules,so that the appearance of transition seeds can be ensured, and a quicktransition from splay alignment to bend alignment can be achieved.

According to a thirty-seventh aspect of the invention, an active matrixliquid crystal display device including an array substrate; an opposingsubstrate; and a liquid crystal layer arranged between the arraysubstrate and the opposing substrate; wherein pretilt angles of theliquid crystal at an upper and at a lower boundary of liquid crystallayer have opposite signs, and in a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process, theliquid crystal is in splay alignment when no voltage is applied;wherein, before liquid crystal display driving, an initializationprocess for a transition from splay alignment to bend alignment isperformed by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by theinitialization;

includes in each pixel a transition-inducing transversal electric fieldapplication portion; and

each pixel includes a first alignment region, wherein a pretilt angle ofliquid crystal molecules in one region at a pixel electrode is a firstpretilt angle, and a pretilt angle of liquid crystal molecules in theone region at a common electrode opposing the pixel electrode is asecond pretilt angle larger than the first pretilt angle; and

a second alignment region, wherein a pretilt angle of liquid crystalmolecules in another region of the pixel electrode is a third pretiltangle, and a pretilt angle of liquid crystal molecules in the otherregion of a common electrode opposing the pixel electrode is a fourthpretilt angle smaller than the third pretilt angle.

The following effects can be attained with this configuration.

Due to the effect of the transversal electric field applicationportions, the pretilt angle in the first alignment region differs fromthe pretilt angle the second alignment region, so that a disclinationline is formed between the first alignment region and the secondalignment region. This disclination line becomes the starting point forthe alignment transition, so that the transition from splay alignment tobend alignment is enhanced.

According to a thirty-eighth aspect of the invention, a liquid crystaldisplay device as in the twenty-ninth aspect further includes a pulsevoltage application portion for applying to the common electrode and thepixel electrode a pulse-shaped voltage with a frequency the range of 0.1Hz to 100 Hz, and a duty ratio in the range of at least 1:1 to 1000:1.

The following effects can be attained with this configuration.

Although there may be certain differences depending for example on size,shape and thickness of the liquid crystal layer, the frequency and theduty ratio of the pulse voltage application portion are limited to theabove ranges so as to enlarge the regions of transition from splayalignment to bend alignment.

Applying such a pulse-shaped second voltage and alternating voltageapplication periods and periods in which no voltage is applied, theliquid crystal molecules are disturbed and transition more readily, sothat the splay aligned liquid crystal molecules transition into bendalignment. Frequency and duty ratio are limited to the above ranges toenlarge the transition regions of transition from splay alignment tobend alignment.

According to a thirty-ninth aspect of the invention, a liquid crystaldisplay device including a pair of substrates; a liquid crystal layerdisposed between the substrates; and a phase compensator arranged on anouter side of the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by thisinitialization;

includes at least one region in the display pixels where the liquidcrystal layer thickness is smaller than around it, and the strength ofan electric field applied to the liquid crystal layer in this region islarger than the strength of an electric field applied to the liquidcrystal layer around it.

With this configuration, more transition seeds appear at the portionswhere the electric field is strong, so that the transition time can beshortened.

According to a fortieth aspect of the invention, a liquid crystaldisplay device including a pair of substrates; a liquid crystal layerdisposed between the substrates; and a phase compensator arranged on anouter side of the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; and wherein the liquid crystaldisplay driving is performed in the bend alignment attained by thisinitialization;

includes at least one region outside the display pixels where the liquidcrystal layer thickness is small, and the strength of an electric fieldapplied to the liquid crystal layer in this region is larger thanstrength of an electric field applied to the liquid crystal layer in thepixels.

With this configuration, electric field concentrations occur outside thepixels, and the transition seeds appearing outside the pixels arepropagated into the pixels. Thus, also in this case, the transition timecan be shortened.

According to a forty-first aspect of the invention, a liquid crystaldisplay device including a pair of substrates; a liquid crystal layerdisposed between the substrates; and a phase compensator arranged on anouter side of the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by thisinitalization;

includes at least one location in the display pixels where the electricfield concentrates.

According to a forty-second aspect of the invention, in a liquid crystaldisplay device as in the forty-first aspect, the location in the displaypixels where the electric field concentrates is at a portion of eitherthe display electrode or the common electrode that partially protrudesin thickness direction of the liquid crystal layer, or both.

Thus, electric field concentrations can be achieved with such aprotruding display electrode configuration.

According to a forty-third aspect of the invention, a liquid crystaldisplay device including a pair of substrates; a liquid crystal layerdisposed between the substrates; and a phase compensator arranged on anouter side of the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by thisinitialization;

includes at least one location outside the display pixels where theelectric field concentrates.

Providing such electric field concentration portions outside the displaypixels, the transition seeds appearing outside the pixels propagate intothe pixels. Thus, also in this case, the transition time can beshortened.

According to a forty-fourth aspect of the invention, in a liquid crystaldisplay device as in the forty-third aspect, the location where theelectric field concentrates is a portion of an electrode that partiallyprotrudes in thickness direction of the liquid crystal layer.

According to a forty-fifth aspect of the invention, a liquid crystaldisplay device including a pair of substrates; a liquid crystal layerdisposed between the substrates; and a phase compensator arranged on anouter side of the substrates; wherein, when no voltage is applied,pretilt angles of the liquid crystal at an upper and at a lower boundaryof the liquid crystal layer have opposite signs, and the liquid crystallayer is in splay alignment, having been subjected to a parallelalignment process; wherein, before liquid crystal display driving, aninitialization process is performed, in which the alignment of theliquid crystal layer is transitioned from splay alignment to bendalignment by application of a voltage; wherein the liquid crystaldisplay driving is performed in the bend alignment attained by thisinitialization; and

a portion of either the display electrode or the common electrode orboth is provided with an aperture portion.

Also with this configuration, the transition time can be shortened.

According to a forty-sixth aspect of the invention, a liquid crystaldisplay device as in the forty-fifth aspect is an active matrix liquidcrystal display device provided with switching elements, and wherein theaperture portion is a conducting via hole electrically connecting pixelelectrodes formed on a flattening film and the switching elements.

Also with this configuration, the transition time can be shortened.

According to a forty-seventh aspect of the invention, in a liquidcrystal display device as in the thirty-ninth aspect, the phasecompensator includes at least one phase compensator made of an opticalmedium with negative reflective index anisotropy whose main axes are inhybrid arrangement.

According to a forty-eighth aspect of the invention, in a liquid crystaldisplay device as in the forty-seventh aspect, the phase compensatorincludes at least one positive phase compensator.

According to a forty-ninth aspect of the invention, a method for drivinga liquid crystal display device includes applying an electric field to aliquid crystal disposed between a first substrate and a second substratearranged in opposition, and transitioning the alignment of the liquidcrystal into bend alignment;

wherein the splay elastic constant k₁₁ of the liquid crystal is in therange of 10×10⁻⁷ dyn≧k₁₁≧6×10⁻⁷ dyn; and

satisfying the relation 1.57 rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is theabsolute value of a pretilt angle of the liquid crystal with respect tothe first substrate and θ₂ is the absolute value of a pretilt angle ofthe liquid crystal with respect to the second substrate.

With this configuration, it is possible to decrease the criticalelectric field for liquid crystal transition, and achieve a quicktransition from the initial alignment of the liquid crystal molecules tothe bend alignment.

According to a fiftieth aspect of the invention, a method for driving aliquid crystal display device including applying an electric field to aliquid crystal disposed between a first substrate and a second substratearranged in opposition, and transitioning the alignment of the liquidcrystal into bend alignment;

wherein the splay elastic constant k₁₁ of the liquid crystal is in therange of 10×10⁻⁷ dyn≧k₁₁≧6×10⁻⁷ dyn; and

wherein the electric field is a main electric field E₀ applied uniformlyover space, to which a secondary electric field E₁ applied non-uniformlyover space is superimposed, satisfying the relation 1.0>E₁−E₀>1/100.

Also with this configuration, it is possible to decrease the criticalelectric field for liquid crystal transition, and achieve a quicktransition from the initial alignment of the liquid crystal molecules tothe bend alignment.

According to a fifty-first aspect of the invention, a method for drivinga liquid crystal display device including applying an electric field toa liquid crystal disposed between a first substrate and a secondsubstrate arranged in opposition, and transitioning the alignment of theliquid crystal into bend alignment;

satisfying the relation 1.57 rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is theabsolute value of a pretilt angle of the liquid crystal with respect tothe first substrate and θ₂ is the absolute value of a pretilt angle ofthe liquid crystal with respect to the second substrate; and

wherein the electric field is a main electric field E₀ applied uniformlyover space, to which a secondary electric field E₁ applied non-uniformlyover space is superimposed, satisfying the relation 1.0>E₁−E₀>1/100.

Also with this configuration, it is possible to decrease the criticalelectric field for liquid crystal transition, and achieve a quicktransition from the initial alignment of the liquid crystal molecules tothe bend alignment.

According to a fifty-second aspect of the invention, a method fordriving a liquid crystal display device including applying an electricfield to a liquid crystal disposed between a first substrate and asecond substrate arranged in opposition, and transitioning the alignmentof the liquid crystal into bend alignment;

wherein the splay elastic constant k₁₁ of the liquid crystal is in therange of 10×10⁻⁷ dyn≧k₁₁≧6×10⁻⁷ dyn; and

satisfying the relation 1.57 rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is theabsolute value of a pretilt angle of the liquid crystal with respect tothe first substrate and θ₂ is the absolute value of a pretilt angle ofthe liquid crystal with respect to the second substrate; and

wherein the electric field is a main electric field E₀ applied uniformlyover space, to which a secondary electric field E₁ applied non-uniformlyover space is superimposed, satisfying the relation 1.0>E₁−E₀>1/100.

Also with this configuration, it is possible to decrease the criticalelectric field for liquid crystal transition, and achieve a quicktransition from the initial alignment of the liquid crystal molecules tothe bend alignment.

Here, the pretilt angle is the alignment angle of the liquid crystalmolecules at the substrate surfaces before the application of anelectric field, representing the tilt of the molecular axis of theliquid crystal molecules at the substrates surfaces with respect to aplane parallel to the substrates over a range of −π/2 to π/2 rad, and ispositive in counter-clockwise direction, taking the plane parallel tothe substrates as the reference (=0). Furthermore, the pretilt angle ofthe liquid crystal at the first substrate is marked with an oppositesign to the pretilt angle of the liquid crystal at the second substrate.

According to a fifty-third aspect of the invention, in a method fordriving a liquid crystal display device as in the fiftieth aspect, thesecondary electric field is applied between a source electrode or a gateelectrode of a thin film transistor formed on a surface of the firstsubstrate, and a transparent electrode formed on a surface of the secondsubstrate.

According to a fifty-fourth aspect of the invention, in a method fordriving a liquid crystal display device as in the fiftieth aspect, thesecondary field is an ac electric field whose oscillation is dampenedover time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing a portion of a liquid crystaldisplay device provided with a bend alignment OCB cell.

FIG. 2 is a cross-sectional view of a liquid crystal cell, illustratingthe transition from splay alignment to bend alignment.

FIG. 3 is a diagram illustrating the configuration of one pixel unit forthe method of driving a liquid crystal display device in accordance witha first embodiment of the present invention.

FIG. 4 illustrates the voltage pattern for alignment transition used inthe first embodiment of the present invention.

FIG. 5 is a graph showing the transition time as a function of the biasvoltage in the first embodiment of the present invention.

FIG. 6 is a diagram illustrating the configuration of one pixel unit forthe method of driving a liquid crystal display device in accordance witha second embodiment of the present invention.

FIG. 7 illustrates the voltage pattern for alignment transition used inthe second embodiment of the present invention.

FIG. 8 is a graph showing the transition time as a function of the biasvoltage in the second embodiment of the present invention.

FIG. 9 is a diagram illustrating the configuration of one pixel unit forthe method of driving a liquid crystal display device in accordance witha third embodiment of the present invention.

FIG. 10 illustrates the voltage pattern for alignment transition used inthe third embodiment of the present invention.

FIG. 11 is a graph showing the transition time as a function of the biasvoltage in the third embodiment of the present invention.

FIG. 12 is a diagram illustrating the configuration of one pixel unitfor the method of driving a liquid crystal display device in accordancewith the third embodiment of the present invention.

FIG. 13 illustrates the regular driving voltage pattern of the liquidcrystal display device in a fourth embodiment of the present invention.

FIG. 14 illustrates the voltage pattern for alignment transition used inthe fourth embodiment of the present invention.

FIG. 15 illustrates the voltage pattern for alignment transition used ina fifth embodiment of the present invention.

FIG. 16 is a diagrammatic cross-sectional view of a liquid crystaldisplay device in accordance with a seventh embodiment of the presentinvention.

FIG. 17 is a diagrammatic plan view of a liquid crystal display devicein accordance with the seventh embodiment of the present invention.

FIG. 18 illustrates a method for manufacturing the liquid crystaldisplay device in accordance with the seventh embodiment of the presentinvention.

FIG. 19 illustrates a liquid crystal display device in accordance withthe eighth embodiment of the present invention. FIG. 19(a) is adiagrammatic cross-sectional view of the liquid crystal display device,and 19(b) is a diagrammatic plan view of the liquid crystal displaydevice.

FIG. 20 schematically illustrates the configuration of a liquid crystaldisplay device in accordance with the ninth embodiment of the presentinvention. FIG. 20(a) is a diagrammatic plan view of the liquid crystaldisplay device, and 20(b) is a diagrammatic cross-sectional view of theliquid crystal display device.

FIG. 21 also schematically illustrates the configuration of a liquidcrystal display device in accordance with the ninth embodiment of thepresent invention.

FIG. 22 shows another example of a liquid crystal display device inaccordance with the ninth embodiment of the present invention.

FIG. 23 schematically illustrates the configuration of a liquid crystaldisplay device in accordance with the tenth embodiment of the presentinvention. FIG. 23(a) is a diagrammatic plan view of the liquid crystaldisplay device, FIG. 23(b) is a diagrammatic cross-sectional view of theliquid crystal display device, FIG. 23(c) is a diagrammaticcross-sectional view of another example of the liquid crystal displaydevice, and FIG. 23(d) is a diagrammatic cross-sectional view of yetanother example of the liquid crystal display device.

FIG. 24 schematically illustrates the configuration of a liquid crystaldisplay device in accordance with an eleventh embodiment of the presentinvention. FIG. 24(a) is a diagrammatic plan view of the liquid crystaldisplay device, and 24(b) is a diagram illustrating how the electricfield ends.

FIG. 25 schematically illustrates the configuration of a liquid crystaldisplay device in accordance with a twelfth embodiment of the presentinvention. FIG. 25(a) is a diagrammatic cross-sectional view of theliquid crystal display device, and 25(b) is a diagrammatic plan view.

FIG. 26 schematically illustrates the cross-sectional configuration of aliquid crystal display device in accordance with a thirteenth embodimentof the present invention.

FIG. 27 illustrates a process for manufacturing bump-shaped protrusionsformed on a glass substrate in liquid crystal display devices of thethirteenth and fourteenth embodiment of the present invention.

FIG. 28 illustrates the continuation of the process for manufacturingbump-shaped protrusions in FIG. 27 in accordance with the presentinvention.

FIG. 29 illustrates the rubbing direction on the substrate used in thethirteenth embodiment of the present invention.

FIG. 30 is a diagram of the configuration in a fourteenth embodiment.

FIG. 31 is a plan view of the fourteenth embodiment.

FIG. 32 is a diagram of the configuration of a test cell used to testthe splay—bend transition time in a liquid crystal display device in afifteenth embodiment of the present invention.

FIG. 33 illustrates a process for manufacturing bump-shaped protrusionsin the liquid crystal display device of the fifteenth embodiment of thepresent invention.

FIG. 34 schematically illustrates the cross-sectional configuration of aliquid crystal display device in a sixteenth embodiment of the presentinvention.

FIG. 35 schematically illustrates the pattern of the transparentelectrodes used in the liquid crystal display device of the sixteenthembodiment.

FIG. 36 is a cross-sectional view showing the main portions of a liquidcrystal display device in accordance with a seventeenth embodiment.

FIG. 37 is a partial magnification of FIG. 36.

FIG. 38 is a cross-sectional view showing the main portions of a liquidcrystal display device in accordance with an eighteenth embodiment.

FIG. 39 illustrates the orientation of the optical elements in theliquid crystal cells used in the liquid crystal display device of theeighteenth embodiment.

FIG. 40 shows the voltage—transmissivity characteristics of the liquidcrystal cells used in the liquid crystal display device of theeighteenth embodiment.

FIG. 41(a) is a schematic diagram illustrating homogenous alignment, andFIG. 41(b) is a schematic diagram illustrating bend alignment.

FIG. 42 is a diagram illustrating the directors of the liquid crystallayer.

FIG. 43 is a schematic of an equivalent CR circuit.

FIG. 44 shows the temporal change of the alignment angle (θj) of theliquid crystal when the external field is increased with time.

FIG. 45 shows the critical electric field (Ec) as a function of thesplay elastic constant (k₁₁).

FIG. 46 shows the critical electric field (Ec) as a function of thedifference (Δθ) of the absolute values of the pretilt angles.

FIG. 47 shows the critical electric field (Ec) as a function of thenon-uniformity (E₁/E₀) of the electric field.

FIG. 48 is a cross-sectional view of a conventional example.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is based on results from observations maderegarding a mechanism for a transition from splay alignment to bendalignment (explained below) in a liquid crystal display device providedwith OCB cells of the bend alignment type. Thus, the specifics of thepresent invention will be explained in the embodiments after explainingthis transition mechanism in detail.

FIG. 1 is a perspective view showing a portion of a liquid crystaldisplay device provided with OCB cells of the bend alignment type. FIG.1 illustrates the configuration of this liquid crystal display deviceprovided with OCB cells of the bend alignment type: A liquid crystallayer 13 including liquid crystal molecules 12 is disposed betweensubstrates 10 and 11 arranged in parallel. Display electrodes (not shownin the drawings) for applying an electric field to the liquid crystallayer 13 and alignment films (not shown in the drawings) governing thealignment of the liquid crystal molecules are formed on opposingsurfaces of the substrates 10 and 11. The alignment films pretilt theliquid crystal molecules 12 at the boundary to the substrate for ca. 5to 7° as shown in FIG. 1, and are subjected to an alignment processleading to the same alignment orientation within the substrate plane,that is, providing parallel orientation. Further away from the surfacesof the substrates 10 and 11, the liquid crystal molecules 12 graduallyrise upright, leading to a bend alignment, in which the tilt angle ofthe liquid crystal molecules is 90° at approximately the center inthickness direction of the liquid crystal layer 13. Polarizers 15 and16, and optical compensators 17 and 18 are arranged on the outer sidesof the substrates 10 and 11. The polarization axes of the two polarizers15 and 16 are arranged at right angles or in parallel, such that theirpolarization axes and the alignment orientation of the liquid crystalmolecules cross at an angle of 45°. Then, using the difference in therefractive index anisotropy in the liquid crystal layer between the ONstate in which a high voltage is applied and the OFF state, in which alow voltage is applied, display is carried out while changing thepolarization with the polarizers and the optical compensators, andcontrolling the transmissivity.

In this liquid crystal display device provided with OCB cells of thebend alignment type, the liquid crystal layer has a splay alignmentbefore use, so that before operating the liquid crystal display, theliquid crystal layer has to be transitioned from splay alignment to bendalignment.

FIG. 2 schematically shows a mechanism for an alignment transition, inwhich the splay alignment of the liquid crystal layer is transitioned toa bend alignment by applying a voltage that is larger than the criticaltransition voltage.

FIG. 2 shows cross-sectional views of liquid crystal cells,schematically illustrating the liquid crystal molecules anddiagrammatically showing the arrangement of the liquid crystal moleculeswhen the two substrates are aligned in parallel.

FIG. 2(a) shows the initial splay arrangement. When there is no electricfield between the substrates, the liquid crystal molecules 12 take on asplay alignment with low energy, in which the major axes of the liquidcrystal molecules 12 in the center of the liquid crystal layer 13 areapproximately parallel to the substrate planes. Here, the liquid crystalmolecules that are parallel to the substrate are marked by the referencenumeral 12 a for illustrative reasons.

FIG. 2(b) shows the arrangement of the liquid crystal molecules whenstarting to apply a high voltage to the electrodes (not shown in thedrawings) formed on the substrates 10 and 11. The electric field startsto tilt slightly the liquid crystal molecules 12 in the center of theliquid crystal layer 13, and as a result, the liquid crystal molecules12 a that are parallel to the substrate planes shift toward one of thesubstrates (in FIG. 2, toward the substrate 11).

FIG. 2(c) shows the arrangement of the liquid crystal molecules aftermore time has passed after applying the voltage. The liquid crystalmolecules 12 at the center of the liquid crystal layer 13 tilt furtherwith respect to the substrate planes, and the liquid crystal molecules12 a that are approximately parallel to the substrate planes are nowcloser to the substrate boundary, where they are subjected to the strongregulative force of the alignment film.

FIG. 2(d) shows the arrangement when the liquid crystal molecules havetransitioned to bend alignment, with an even higher energy state. Theliquid crystal molecules 12 at the center of the liquid crystal layer 13are now perpendicular with respect to the substrate planes, and theliquid crystal molecules contacting the boundary of the alignment film(not shown in the drawings) on the substrate 10 are subjected to thestrong regulative force of the alignment film, so that they retain theiroblique orientation, and there are almost no more liquid crystalmolecules 12 a arranged in parallel to the substrate planes, as therewere in FIGS. 2(a) to (c).

When even more time as passed from FIG. 2(d), the alignment between thesubstrates shifts to the bend alignment shown in FIG. 1, ending thetransition.

It is believed that these are the circumstances under which thetransition from splay alignment to bend alignment takes place whenapplying a voltage.

However, usually when the transition occurs, it does not occur at oncein the entire liquid crystal layer between the substrates, buttransition seeds occur around spacers distributed throughout the gap orat alignment irregularities (that is, at portions of the alignmentregion, at which energy transitions are easier), from which the bendalignment region spreads. Consequently, in order to achieve an alignmenttransition in the OCB cells, it is necessary to create transition seedsin at least some of the regions of the liquid crystal layer between thesubstrates, and, applying energy from the outside, to make a shift fromsplay alignment to the bend alignment with higher energy, and tomaintain this bend alignment.

Examining this mechanism of alignment transition, the inventorsconceived of a liquid crystal display device in which transition seedsappear reliably and in which transitions can be accomplished in veryshort time, as well as a method for manufacturing such a liquid crystaldisplay device and a method for driving a liquid crystal display device.The specifics of the invention are explained under reference to theembodiments.

First Embodiment

FIG. 3 is a diagram illustrating the configuration for one pixel unit inthe method of driving a liquid crystal display device in accordance witha first embodiment of the present invention. First, the configuration ofa liquid crystal display device associated with the driving method ofthe first embodiment is explained with reference to FIG. 3. Except forthe configuration of the driving circuit, the liquid crystal displaydevice of the first embodiment has the same configuration as a liquidcrystal display device provided with regular OCB cells. That is to say,it has a pair of glass substrates 20 and 21, and a liquid crystal layer26 disposed between the glass substrates 20 and 21. The glass substrates20 and 21 are arranged in opposition with a predetermined spacingbetween them. A common electrode 22 made of transparent ITO is formed onthe inner side of the glass substrate 20 and a pixel electrode 23 madeof transparent ITO is formed on the inner side of the glass substrate21. Alignment films 24 and 25 made of polyimide are formed on the commonelectrode 22 and the pixel electrode 23, and these alignment films 24and 25 are subjected to an alignment process, for arranging theiralignment directions in parallel. Then, a p-type nematic liquid crystal26 is disposed between the alignment films 24 and 25. The pretilt angleof the liquid crystal molecules on the alignment films 24 and 25 is setto about 5°, and the critical voltage for transition from splayalignment to bend alignment is set to 2.5V. The retardation of theoptical compensator 29 is selected such that the display is either whiteor black during the ON state. The numerals 27 and 28 in FIG. 1 denotepolarizers.

In FIG. 3, numeral 30 denotes an alignment transition driving circuitand numeral 31 denotes a liquid crystal display driving circuit.Numerals 32 a and 32 b denote switching circuits, and numeral 33 denotesa switching control circuit for controlling the switching of theswitching circuits 32 a and 32 b. The switching circuit 32 a includestwo individual contact points P1, P2, and a common contact point Q1, andthe switching circuit 32 b includes two individual contact points P3,P4, and a common contact point Q2. The common contact point Q1 isconnected to either the individual contact point P1 or to P2 dependingon the switching signal S1 from the switching control circuit 33.Similarly, the common contact point Q2 is connected to either theindividual contact point P3 or to P4 depending on the switching signalS2 from the switching control circuit 33. When the common contact pointQ1 is connected to the individual contact point P1, and the commoncontact point Q2 is connected to the individual contact point P3, thenthe driving voltage from the alignment transition driving circuit 30 isapplied to the electrodes 22 and 23. When the common contact point Q1 isconnected to the individual contact point P2, and the common contactpoint Q2 is connected to the individual contact point P4, then thedriving voltage from the liquid crystal display driving circuit 33 isapplied to the electrodes 22 and 23.

The following is an explanation of a driving method in accordance withthe first embodiment.

Before liquid crystal display driving with a regular image signal, aninitialization process is carried out for transition into bendalignment. First, when turning on the power, the switching controlcircuit 33 outputs the switching signals S1 and S2 to the switchingcircuits 32 a and 32 b, the common contact point Q1 is connected to theindividual contact point P1, and the common contact point Q2 isconnected to the individual contact point P3. Thus, the alignmenttransition driving circuit 30 applies the driving voltage shown in FIG.4 to the electrodes 22 and 23. This driving voltage is an ac voltage, inwhich an ac square voltage A is superimposed with a bias voltage B, asshown in FIG. 4, and the driving voltage is set to a value that islarger than the critical voltage, which is the minimum voltage necessaryfor transition from the splay alignment to the bend alignment. Applyingthis driving voltage, it becomes possible to make the transition timemuch shorter than in the conventional examples that simply apply an acvoltage. The reasons why the transition time is shortened are explainedbelow. Thus, the initialization process for transition into the bendalignment is terminated.

Then, after the transition time for completing the transition over theentire electrode has passed, the switching control circuit 33 outputs tothe switching circuit 32 a a switching signal S1 that switches theconnection of the common contact point Q1 to the individual contactpoint P2, and to the switching circuit 32 b a switching signal S2 thatswitches the connection of the common contact point Q2 to the individualcontact point P4. Thus, the common contact point Q1 is connected withthe individual contact point P2, and the common contact point Q2 isconnected with the individual contact point P4, and applying a drivingsignal voltage with the liquid crystal display driving circuit 31 to theelectrodes 22 and 23, the desired image is displayed. Here, the liquidcrystal display driving circuit 31 applies a 30 Hz square voltage of2.7V sustaining the bend alignment in the OFF state, and a 30 Hz squarevoltage of 7V in the ON state, thus achieving display on the OCB panel.

Then, the inventors produced a liquid crystal display device with thisconfiguration and experimentally performed the initialization processwith this driving method; the results are listed below. The experimentalconditions were as follows:

The electrode area was set to 2 cm², the cell gap to ca. 6 μm, thefrequency of the ac square voltage A to 30 Hz and its amplitude to ±4V.

Under these conditions, the transition times were measured for the fourdifferent bias voltages B of 0V, 2V, 4V and 5V; the results are shown inFIG. 5. Here, “transition time” means the time needed to complete thetransition of the alignment for the entire region of the electrode area.

As becomes clear from FIG. 5, when the bias voltage B is 0V, atransition time of 140 s is needed. On the other hand, when the biasvoltage B is 4V, the transition time can be shortened to only 8 s. Itseems that due to the superimposition of the bias voltage, the alignmentof the liquid crystal molecules in the liquid crystal layer is disturbedby the bias voltage, so that deviations occur between the substrates asshown in FIG. 2(d), leading to many transition seeds, and that theincrease of the effective voltage speeds up the transition time.

Thus, by continuously applying an ac voltage superimposed with a biasvoltage, the transition time can be made shorter than when simplyapplying an ac voltage.

In this experimental example, the ac square voltage signal had afrequency of 30 Hz and an amplitude of ±4V, but the present invention isnot limited to this, and any frequency with which the liquid crystal canbe operated is suitable, such as even 10 kHz, and the transition timecan of course be shortened by increasing the amplitude of the ac voltageA. The transition time is shorter, the higher the superimposed biasvoltage B is. However, striving for lower voltages as driving voltages,it is preferable that the bias voltage is set to a suitable voltagelevel depending on the desired transition time. Furthermore, a squarewave was used as the waveform, but it is also possible to use an acwaveform with a different duty ratio.

Second Embodiment

FIG. 6 is a diagram of the configuration of a liquid crystal displaydevice in accordance with the second embodiment of the present inventionfor one pixel unit. The second embodiment is characterized in thattransitions of the liquid crystal layer from the splay alignment to thebend alignment are caused by repeating, in alternation, a step ofapplying an ac voltage superimposed with a bias voltage between thesubstrates, and a step of putting the substrates into an electricallyreleased state (open state).

Structural elements of the liquid crystal display device in accordancewith the second embodiment that correspond to those in the liquidcrystal display device of the first embodiment have been denoted by thesame reference numerals and their further explanation has been omitted.Instead of the alignment transition driving circuit 30, the switchingcircuit 32 a and the switching control circuit 32 of the firstembodiment, the second embodiment includes an alignment transitiondriving circuit 40, a switching circuit 42 a and a switching controlcircuit 43. The switching circuit 42 a is a three-way switching circuitincluding an individual contact point P5 in addition to the individualcontact points P1 and P2. The switching circuit 43 controls theswitching of this switching circuit 42 a. The alignment transitiondriving circuit 40 applies the driving voltage shown in FIG. 7 betweenthe substrates 22 and 23. As shown in FIG. 7, this driving voltage is anac voltage, in which an ac square voltage C is superimposed with a biasvoltage D, and whose amplitude is set to a voltage that is larger than acritical voltage, which is the minimum voltage necessary for transitionfrom the splay alignment to the bend alignment.

The common contact point Q1 of the switching circuit 42 a is connectedto one of the individual contact points P1, P2 or P5, depending on theswitching signal S3 from the switching control circuit 42. Connectingthe common contact point Q1 to the individual contact point P5 leads toan open state, in which the electrodes 22 and 23 are disconnected fromthe alignment transition driving circuit 40. If the common contact pointQ1 is connected to the individual contact point P1 and the commoncontact point Q2 is connected to the individual contact point P3, thenthe driving voltage from the alignment transition driving circuit 40 isapplied to the electrodes 22 and 23. If the common contact point Q1 isconnected to the individual contact point P2 and the common contactpoint Q2 is connected to the individual contact point P4, then thedriving voltage from the liquid crystal display driving circuit 31 isapplied to the electrodes 22 and 23.

The following is an explanation of a driving method in accordance withthe second embodiment.

Before liquid crystal display driving with a regular image signal, aninitialization process is carried out for transition into bendalignment. First, when turning on the power, the switching controlcircuit 43 outputs a switching signal S3 to the switching circuit 42 aand a switching signal S2 to the switching circuit 32 b, the commoncontact point Q1 is connected to the individual contact point P1, andthe common contact point Q2 is connected to the individual contact pointP3. Thus, the alignment transition driving circuit 30 applies thedriving voltage shown in FIG. 7 to the electrodes 22 and 23. Then, aftera predetermined period of time T2 has passed, the switching controlcircuit 43 outputs a switching signal S3 to the switching circuit 42 a,and the common contact point Q1 is connected to the individual contactpoint P5. This leads to an open state, in which the electrodes 22 and 23are disconnected from the alignment transition driving circuit 40. Thisopen state is maintained for a period W2, during which the chargebetween the electrodes 22 and 23 is held.

When the period W2 in the open state has passed, the switching controlcircuit 43 outputs a switching signal S3 to the switching circuit 42 a,and the common contact point Q1 is again connected with the individualcontact point P1. Then, this alignment transition driving and the openstate are repeated in alternation, and after a certain period of timehas passed after turning on power, the entire area of the electrode hastransitioned into bend alignment.

Then, after this period of time has passed, the switching controlcircuit 43 outputs a switching signal S3 to the switching circuit 42 aand a switching signal S2 to the switching circuit 32 b, so that thecommon contact point Q1 is connected to the individual contact P2 andthe common contact point Q2 is connected to the individual contact P43,so that the liquid crystal display driving circuit 31 applies a drivingsignal voltage to the electrodes 20 and 21, and the desired image isdisplayed. Here, as in the first embodiment, the liquid crystal displaydriving circuit 31 applies a 30 Hz square voltage of 2.7V sustaining thebend alignment in the OFF state, and a 30 Hz square voltage of 7V in theON state, thus achieving display on the OCB panel.

Then, the inventors produced a liquid crystal display device with thisconfiguration and experimentally performed the initialization processwith this driving method; the results are listed below. The experimentalconditions were as follows:

The electrode area was set to 2 cm², the cell gap to ca. 6 μm, the biasvoltage B was set to 2V, the frequency of the ac square voltage D to 30Hz and its amplitude to ±4V, and the application time T2 was set to 2 s.

Under these conditions, the transition times were measured with 0 s, 0.2s, 2 s and 3 s for the open state period W2, repeating the voltageapplication state and the open state in alternation. The results areshown in FIG. 8. Here, “transition time” means the time needed tocomplete the transition of the alignment for the entire region of theelectrode area.

As becomes clear from FIG. 8, when the open state period W2 was 0 s,that is, when the ac voltage superimposed with the bias voltage wasapplied continuously, a transition time of 80 s was needed. When, on theother hand, the open state period W2 was 0.2 s, and alternated with theac voltage with the superimposed bias voltage, then the transition timewas shortened to 40 s. However, when the open state period W2 was 2 s,then the transition time became 420 s long, and when W2 was 3 s, thenthe transition could not be finished.

Furthermore, when the application time T2 was set to 0.3 s and the openstate period W2 to 0.3 s, and the remaining conditions were as in theabove example, a transition time of 28 s was measured.

Favorable results were obtained when T2 was set to 2 s, and W2 was atleast 0.1 s but not greater than 0.5 s.

It seems that the shortening of the time for shifting from splayalignment to bend alignment by repeating the biased ac voltage and theopen state in this manner is due to the following reasons: Applying anac voltage with a superimposed bias, the liquid crystal moleculealignment of the liquid crystal layer is disturbed, and deviations occurbetween the substrates as shown in FIG. 2(d), and transition seeds aregenerated by switching then to a short open state, shortening thetransition time.

This effect can also be attained when applying yet another voltagesignal before or after the step of applying the ac voltage with thesuperimposed bias, and then going into the open state.

Also, the bias voltage and the ac voltage, the application time, and thetime that the open state is maintained can be chosen as appropriate forthe desired transition time. The frequency of the ac voltage should be afrequency at which the liquid crystal can be operated, such as even 10kHz. A square wave was used as the waveform, but it is also possible touse an ac waveform with a different duty ratio.

Third Embodiment

FIG. 9 is a diagram of the configuration of a liquid crystal displaydevice in accordance with the third embodiment of the present inventionfor one pixel unit. The third embodiment is characterized in thattransitions of the liquid crystal layer from the splay alignment to thebend alignment are caused by repeating, in alternation, a step ofapplying an ac voltage superimposed with a bias voltage to thesubstrates, and a step of applying no voltage or a low voltage to thesubstrates.

Structural elements of the liquid crystal display device in accordancewith the third embodiment that correspond to those in the liquid crystaldisplay device of the second embodiment have been denoted by the samereference numerals and their further explanation has been omitted.

Instead of the switching circuit 32 b and the switching control circuit43 of the second embodiment, the third embodiment uses a switchingcircuit 42 b and a switching control circuit 53. Also, in addition tothe alignment transition driving circuit 40, the third embodiment isprovided with an alignment transition driving circuit 50 for applying alow voltage to the electrodes 22 and 23.

The switching circuit 42 b is a three-way switching circuit including anindividual contact point P6 in addition to the individual contact pointsP3 and P4. The switching circuit 53 controls the switching of thisswitching circuit 42 b. The switching signal S4 from the switchingcontrol circuit 53 connects the common contact point Q2 of the switchingcircuit 42 b to the individual contact points P3, P4, and P6.

When the common contact point Q1 is connected to the individual contactpoint P5 and the common contact point Q2 is connected to the individualcontact point P3, then the driving voltage from the alignment transitiondriving circuit 40 is applied to the electrodes 22 and 23. When thecommon contact point Q1 is connected to the individual contact point P5and the common contact point Q2 is connected to the individual contactpoint P6, then the driving voltage from the alignment transition drivingcircuit 50 is applied to the electrodes 22 and 23. Furthermore, when thecommon contact point Q1 is connected to the individual contact point P2and the common contact point Q2 is connected to the individual contactpoint P4, then the driving voltage from the liquid crystal displaydriving circuit 31 is applied to the electrodes 22 and 23.

The following is an explanation of a driving method in accordance withthe third embodiment.

Before liquid crystal display driving with a regular image signal, aninitialization process is carried out for transition into bendalignment. First, when turning on the power, the switching controlcircuit 53 outputs a switching signal S3 to the switching circuit 42 aand a switching signal S4 to the switching circuit 42 b, the commoncontact point Q1 is connected to the individual contact point P1, andthe common contact point Q2 is connected to the individual contact pointP3. Thus, the alignment transition driving circuit 40 applies thedriving voltage shown in FIG. 10 to the electrodes 22 and 23. Then,after a predetermined period of time T3 has passed, the switchingcontrol circuit 53 outputs a switching signal S3 to the switchingcircuit 42 a and a switching signal S4 to the switching circuit 42 b, sothe common contact point Q1 is connected to the individual contact pointP5, and the common contact point Q2 is connected to the individualcontact point P6. Thus, the low voltage shown in FIG. 10 is applied bythe alignment transition driving circuit 50 to the electrodes 22 and 23.This low voltage stays applied for a period W3.

When the low voltage application period W3 has passed, the switchingcontrol circuit 53 outputs a switching signal S3 to the switchingcircuit 42 a and a switching signal S4 to the switching circuit 42 b, sothe common contact point Q1 is again connected with the individualcontact point P1 and the common contact point Q2 is again connected withthe individual contact point P3. Then, this ac voltage application stepand the low-voltage application step are repeated in alternation, andafter a certain period of time has passed after turning on power, theentire area of the electrode has passed into bend alignment.

Then, after this period of time has passed, the switching controlcircuit 53 outputs a switching signal S3 to the switching circuit 42 aand a switching signal S4 to the switching circuit 42 b, so that thecommon contact point Q1 is connected to the individual contact P2 andthe common contact point Q2 is connected to the individual contact P43.Thus, the liquid crystal display driving circuit 31 applies a drivingsignal voltage to the electrodes 20 and 21, and the desired image isdisplayed. Here, as in the first embodiment, the liquid crystal displaydriving circuit 31 applies a 30 Hz square voltage of 2.7V sustaining thebend alignment in the OFF state, and a 30 Hz square voltage of 7V in theON state, thus achieving display on the OCB panel.

Then, the inventors produced a liquid crystal display device with thisconfiguration and experimentally performed the initialization processwith this driving method; the results are listed below. The experimentalconditions were as follows:

The electrode area was set to 2 cm², the cell gap to ca. 6 μm, the biasvoltage D was set to 2V, the frequency of the ac square voltage C to 30Hz and its amplitude to ±4V, and the application time T3 was set to 1 s.Furthermore, the voltage applied during the low voltage applicationperiod W3 was set to a dc voltage of −2V.

Under these conditions, the transition times were measured for severallow voltage application periods W3, repeating the voltage applicationstate and the application voltage application state in alternation. Theresults are shown in FIG. 11.

As becomes clear from FIG. 11, when the low voltage application time is0 s, that is, when the ac voltage superimposed with the bias voltage wasapplied continuously, a transition time of ca. 80 s was needed. When, onthe other hand, the low voltage application period W3 was set to 0.1 sand repeated in alternation with the ac voltage superimposed with thebias voltage, then the transition time was shortened to 60 s. However,when the low voltage application period W3 was set to 1 s, then thetransition time became 360 s long, and when W3 was set to 3 s, then thetransition could not be finished.

Furthermore, repeatedly switching between an ac voltage of ±4Vsuperimposed with a bias voltage of 2V and a dc voltage of 0V, thetransition was completed within 50 s in the shortest case. Also,repeatedly switching between an ac voltage of ±4V superimposed with abias voltage of 2V and an ac voltage of ±2V, the transition wascompleted within 50 s in the shortest case.

Favorable results were obtained when T3 was set to 1 s, and W2 was atleast 0.1 s but not greater than 0.5 s.

The transition time for shifting from splay alignment to bend alignmentwhen repeating application of the biased ac voltage and of the lowvoltage is shorter than when simply applying a continuous ac voltagesuperimposed with a bias voltage. It seems that this is, becauseapplying an ac voltage with a bias superimposed on it, the liquidcrystal molecule alignment in the liquid crystal layer is disturbed, anddeviations occur between the substrates as shown in FIG. 2(d), andtransition seeds are created by switching then to a short low voltageapplication state, shortening the transition time.

Also, the bias voltage and the ac voltage, and their application time,as well as the low voltage and its application time can be chosen asappropriate for the desired transition time. The frequency of the acvoltage should be a frequency at which the liquid crystal can beoperated, such as 10 kHz. A square wave was used as the waveform, but itis also possible to use an ac waveform with a different duty ratio.

Moreover, in the low voltage application period W3, a low voltage of −2Vwas applied, but it is also possible to apply 0V.

The following is a discussion of the ratio between the ac voltageapplication period T3 and the low voltage application period W3, and thenumber of times the ac voltage application and the low voltageapplication are repeated per second. The voltage during the low voltageapplication period W3 is 0V, and the ac voltage application is repeatedin alternation with this application of 0V, which, for illustrativereasons, is regarded as one transition voltage as indicated by thebroken line L in FIG. 10. In that case, to shorten the transition time,the frequency of the transition voltage L should be set within a regionof 0.1 Hz to 100 Hz and the duty ratio of the transition voltage Lshould be set within a region of 1:1 to 1000:1. It is preferable thatthe frequency of the transition voltage L is within a range of 0.1 Hz to10 Hz, and that the duty ratio of the transition voltage L is set withina region of 2:1 to 1000:1. The following explains the reasons for this.

When the duty ratio of the repeatedly applied voltage is in the range of1:1 to 1:10, transition seeds may appear during the pulses in whichvoltage is applied, but between the pulses, when no voltage is applied,the transition returns to splay alignment within a certain relaxationtime, and the transition never completes. In order to enlarge thetransition region, the duty ratio should be set to a range of 1:1 to1000:1, preferably 2:1 to 100:1, so that the pulse width is broader thanthe interval between pulses. It seems that above 1000:1 towardcontinuous dc, as there is almost no pulse repetition left, there arefewer opportunities for transition seeds to form, and the transitiontakes somewhat longer.

Also, for the repetition frequency of the transition voltageapplication, a frequency between continuous application and about 100 Hzis suitable, but preferable is a frequency from 10 Hz, at which a pulsewidth of at least about 100 ms can be attained for transitionenlargement, to 0.1 Hz, at which a pulse interval of at least about 10ms is attained at a duty ratio of 1000:1.

The inventors measured the transition time applying an alternation avoltage of −15V dc and 0V to the liquid crystal cell and changing therepetition frequency and the duty ratio. The results are shown in Table1.

TABLE 1 frequency duty 0.1 Hz 1 Hz 10 Hz 100 Hz 1:1 300 180 150 220 2:140 30 25 35 10:1  15 10 8 15 100:1  10 10 12 18 1000:1   10 12 15 18 dc20 (unit: sec)

As can be seen from Table 1, when the frequency is in the range of 0.1Hz to 10 Hz and the duty ratio is in the range of 2:1 to 1000:1, thetransition time is very short, and also when the frequency is in therange of 0.1 Hz to 100 Hz and the duty ratio is in the range of 1:1 to1000:1, the transition time is still sufficiently short.

Fourth Embodiment

FIG. 12 is a diagram of the configuration of a liquid crystal displaydevice in accordance with the fourth embodiment of the present inventionfor one pixel unit. The fourth embodiment illustrates an example, inwhich the present invention is applied to a method for driving an activematrix liquid crystal display device.

First, the configuration of a liquid crystal display device associatedwith the driving method of the fourth embodiment is explained withreference to FIG. 12. Except for the configuration of the drivingcircuit, the liquid crystal display device in accordance with the fourthembodiment has the same configuration as an active matrix liquid crystaldisplay device with regular OCB cells. That is to say, it has a pair ofglass substrates 60 and 61, and a liquid crystal layer 66 interposedbetween the glass substrates 60 and 61. The glass substrates 60 and 61are arranged in opposition to one another at a certain spacing. A commonelectrode 62 made of transparent ITO is formed on the inner side of theglass substrate 60, a thin film transistor (TFT) 70 serving as a pixelswitching element and a pixel electrode 63 made of transparent ITO andconnected to the TFT 70 are formed on the inner side of the glasssubstrate 61. Alignment films 64 and 65 of polyimide are formed on thecommon electrode 62 and the pixel electrode 63, and these alignmentfilms 64 and 65 are subjected to an alignment process so as to arrangetheir alignment directions in parallel. Then, a p-type nematic liquidcrystal layer 66 is disposed between the alignment films 64 and 65. Thepretilt angle of the liquid crystal molecules on the alignment films 64and 65 is set to about 50, and the critical voltage for transition fromsplay alignment to bend alignment is set to 2.6V. The retardation of theoptical compensator 67 is selected such that the display is either whiteor black during the ON state. The numerals 67 and 68 in FIG. 12 denotepolarizers.

In FIG. 12, numerals 71 and 72 denote alignment transition drivingcircuits. The function of the alignment transition driving circuit 71 isto apply a driving voltage to the common electrode 62, taking thepotential of the common electrode in FIG. 14 as a center reference, andto apply 0V to the pixel electrode 63. The function of the otheralignment transition driving circuit 72 is to apply 0V to the commonelectrode 62 and the pixel electrode 63. Numeral 73 denotes a liquidcrystal display driving circuit. The function of the liquid crystaldisplay driving circuit 73 is to apply a driving voltage with thevoltage pattern shown in FIG. 13 to the common electrode 62 and thepixel electrode 63. That is to say, the liquid crystal display drivingcircuit 73 applies the voltage marked M1 in FIG. 13 to the pixelelectrode 63, and the voltage marked M2 in FIG. 13 to the commonelectrode 62. This means that in this configuration, during thealignment transition period, 0V is applied to the pixel electrode 63,but it is also possible to apply a pixel electrode voltage with theliquid crystal display driving circuit 73 during the alignmenttransition period.

Numerals 74 a and 74 b denote switching circuits, and numeral 75 denotesa switching control circuit for controlling the switching of theswitching circuits 74 a and 74 b. The switching circuit 74 a includesthree individual contact points P7, P8, P9, and a common contact pointQ1, and the switching circuit 74 b includes three individual contactpoints P10, P11, P12, and a common contact point Q2. When the commoncontact point Q1 is connected to the individual contact point P7, andthe common contact point Q2 is connected to the individual contact pointP10, then the driving voltage from the alignment transition drivingcircuit 71 is applied to the electrodes 62 and 63. When the commoncontact point Q1 is connected to the individual contact point P2, andthe common contact point Q2 is connected to the individual contact pointP4, then the driving voltage from the liquid crystal display drivingcircuit 73 is applied to the electrodes 62 and 63.

The following is an explanation of a driving method in accordance withthe fourth embodiment.

Before liquid crystal display driving with a regular image signal, aninitialization process is carried out for transition into bendalignment. First, when turning on the power, the switching controlcircuit 75 outputs a switching signal to the switching circuit 74 a anda switching signal to the switching circuit 74 b, the common contactpoint Q1 is connected to the individual contact point P7, and the commoncontact point Q2 is connected to the individual contact point P10. Thus,the alignment transition driving circuit 71 applies the driving voltageshown in FIG. 14 to the common electrode 62. This means, an ac voltagesynchronized with the vertical synchronization signal, to which a biasvoltage −GV is superimposed, is applied to the common electrode 62,taking the potential of the common electrode as a center reference. 0Vis applied to the pixel electrode. Then, this ac voltage is maintainedfor the period T4.

Then, after this ac voltage application period T4 has passed, theswitching control circuit 75 outputs a switching signal to the switchingcircuit 74 a and a switching signal to the switching circuit 74 b, suchthat the common contact point Q1 is connected to the individual contactpoint P9, and the common contact point Q2 is connected to the individualcontact point P12. Thus, the alignment transition driving circuit 72applies 0V to the common electrode 62 and the pixel electrode 63, asshown in FIG. 14. This 0V voltage stays applied for a period W4.

When the 0V voltage application period W4 has passed, the switchingcontrol circuit 75 outputs a switching signal to the switching circuit742 a and a switching signal to the switching circuit 74 b, so thecommon contact point Q1 is again connected with the individual contactpoint P7 and the common contact point Q2 is again connected with theindividual contact point P10. Then, this ac voltage application step andthe 0V voltage application step are repeated in alternation, and after acertain period of time has passed after turning on power, the entirearea of the electrode has passed into bend alignment.

Then, after this certain period of time has passed, the switchingcontrol circuit 75 outputs a switching signal to the switching circuit74 a and a switching signal to the switching circuit 74 b, so that thecommon contact point Q1 is connected to the individual contact P8 andthe common contact point Q2 is connected to the individual contact P11.Thus, the liquid crystal display driving circuit 73 applies a drivingsignal voltage to the electrodes 62 and 63, and the desired image isdisplayed. Here, the liquid crystal display driving circuit 73 applies adriving voltage of at least 2.7V sustaining the bend alignment in theOFF state, and a voltage of 7V as an upper limit in the ON state, thusachieving display on the OCB panel.

With this driving method, a high-quality bend alignment display devicewith a broad viewing angle and very fast response was attained, withoutany alignment defects in the OCB active matrix liquid crystal displaydevice.

Then, the inventors produced a liquid crystal display device with thisconfiguration and experimentally performed the initialization processwith his driving method; the results are listed below. The experimentalconditions were as follows:

The cell gap was set to ca. 6 μm, the bias voltage G was set to −6V, thefrequency of the ac square voltage D to 7.92 Hz and its amplitude to±10V, and the application time T3 was set to 0.5 s. Moreover, the 0Vvoltage application period W4 was set to 0.5 s.

According to the experimental results, the alignment transition in allthe pixels in the panel of the liquid crystal display device wasfinished within approximately 2 s.

When no bias voltage was superimposed, then about 20 s were necessaryfor the transition of the alignment of the entire display. Thus, it canbe appreciated that also in the fourth embodiment, the driving with asuperimposed bias voltage accomplishes a shortening of the transitiontime.

Fifth Embodiment

Instead of the driving voltage pattern shown in FIG. 14, it is alsopossible to use the driving voltage pattern in FIG. 15 for driving thealignment transition of an OCB-mode active matrix liquid crystal displaydevice. That is to say, during the ac voltage application period T4, adc voltage of −15V is applied for 0.5 s to the common electrode 62,taking the potential of the common electrode as a center reference.After that, during the 0V voltage application period W4, 0V is appliedfor 0.2 s. Then, the application of −15V dc and the application of 0Vare repeated in alternation. Also with this driving method, thetransition is carried out reliably and in a very short time.

Carrying out experiments with this driving method, the inventors foundthat a transition time of less than 2 s can be attained.

Sixth Embodiment

Instead of the active matrix liquid crystal display device used in thefourth and fifth embodiments, the sixth embodiment applies the drivingmethod of the fourth and fifth embodiments to a liquid crystal displaydevice with a flattened film configuration, in which a flattening filmis arranged on the switching element, and the pixel electrode is formedon the flattening film. To give a specific example of this drivingmethod, the alignment transition voltage superimposed with the biasvoltage in the fourth embodiment was applied for 0.5 s, and then theopen state was held for 0.5 s, which was repeated in alternation. Withthis driving method, the transition went even smoother, with atransition time of less than 1 s. It seems that this is, because withthe flattening film configuration, the pixel electrode interval becomessmaller, so that a smoother transition from the splay alignment to thebend alignment is possible.

Other Considerations

{circle around (1)} In the above embodiments, an ac voltage superimposedwith a bias voltage is applied, but it is also possible to apply a dcvoltage, in which case the driving circuit can be simplified, becauseonly voltages of one polarity are applied. {circle around (2)} In theabove embodiments, in the ac voltage signal superimposed with the biasvoltage, the bias voltage was described as a dc signal, but it is alsopossible to use a low-frequency ac signal in order to increasereliability. {circle around (3)} The optimum ranges for the frequencyand the duty ratio of the repeated voltage can also be applied to theother embodiments besides the third embodiment. {circle around (4)} Inthe above embodiments, the method for driving a liquid crystal displaydevice of the present invention was described for a transmission-typeliquid crystal display device, but it can also be used for areflection-type liquid crystal display device. It can also be used for afull-color liquid crystal display device using a color filter, or aliquid crystal display device without color filter.

Seventh Embodiment

FIG. 16 is a diagrammatic cross-sectional view of a liquid crystaldisplay device in accordance with the seventh embodiment of the presentinvention, and FIG. 17 is a diagrammatic plan view thereof.

The liquid crystal display device shown in FIG. 16 includes polarizers101 and 102, a phase compensator 103 for optical compensation disposedon an inner side of the polarizer 101, and an active matrix liquidcrystal cell 104 arranged between the polarizers 101 and 102.

The liquid crystal cell 104 includes an array substrate 106 made, forexample, of glass, and an opposing substrate 105 in opposition to thearray substrate 106. Transparent pixel electrodes 108 are formed on theinner surface of the array substrate 106, and the common electrode 107is formed on the inner surface of the opposing substrate 105. Moreover,an alignment film 110 is formed on the pixel electrode 108, and analignment film 109 is formed on the pixel electrodes 107.

Switching element 111 made for example of an a-Si TFT are arranged onthe array substrate 106, and these switching element 111 are connectedto the pixel electrodes 108.

Furthermore, spacers of 5 μm diameter (not shown in the drawings) and aliquid crystal layer 112 made of a nematic liquid crystal material withpositive dielectric anisotropy are arranged between the alignment films109 and 110. Moreover, the alignment films 109 and 110 are subjected toa parallel alignment process, such that the pretilt angles of the liquidcrystal molecules on their surfaces have opposite signs, and thealignment films 109 and 110 are arranged substantially in parallel.Consequently, the liquid crystal layer 112 has a so-called splayalignment, in which the liquid crystal molecules are arranged inalignment regions that widen up diagonally when no voltage is applied.

The alignment film 110 includes an alignment film 110 a with a largerpretilt angle B2 (third pretilt angle) and an alignment film 110 b witha smaller pretilt angle A2 (first pretilt angle). Moreover, thealignment film 109 includes an alignment film 109 a with a smallerpretilt angle D2 (fourth pretilt angle) and an alignment film 109 b witha larger pretilt angle C2 (second pretilt angle). The pretilt angle C2is arranged in opposition to the pretilt angle A2, and the pretilt angleD2 is arranged in opposition to the pretilt angle B2.

The orientation films 109 and 110 are subjected to a parallel alignmentprocess by rubbing with a rubbing cloth in a direction substantiallyperpendicular to the signal electrode lines 113, and in the samedirection for the upper and the lower substrate (that is, from left toright in FIG. 16).

The following is an explanation of a method for manufacturing thisliquid crystal display device.

First, signal scanning lines 113, switching elements 111 and pixelelectrodes 108 are formed the inner side of an array electrode 106.

Then, a polyimide alignment film material of polyamic acid type (byNissan Chemical Industries, Ltd.), having a large pretilt angle B2 ofabout 5° as the third pretilt angle, is spread on the pixel electrode108, dried and baked, thereby forming the alignment film 110 a on thepixel electrode 108.

Then, the region on the left side in the paper plane of the alignmentfilm 110 a is irradiated with UV light to reduce the pretilt angle A2(first pretilt angle) by about 2°, thereby forming the alignment film110 b.

The common electrode 107 is formed on the inner side of the opposingsubstrate 105.

Then, a polyimide alignment film material of polyamic acid type (byNissan Chemical Industries, Ltd.) imparting a large pretilt angle C2 ofabout 5° as the second pretilt angle to the liquid crystal molecules atthe boundary is spread on the common electrode 107, dried and baked,thereby forming the alignment film 109 b on the common electrode 107.

Then, the region on the right side in the paper plane of the alignmentfilm 109 b (that is, the region in opposition to the larger pretiltangle B2) is irradiated with UV light to reduce the pretilt angle D2(fourth pretilt angle) by about 2°, thereby forming the alignment film109 a.

In this manner, a smaller pretilt angle A2 (first pretilt angle) isformed in opposition to a larger pretilt angle C2 (second pretiltangle), and a larger pretilt angle B2 (third pretilt angle) is formed inopposition to a smaller pretilt angle D2 (fourth pretilt angle), asshown in FIG. 16 The pretilt angles can also be controlled as follows:

As shown in FIG. 18(a), an active matrix switching element (not shown inthe drawings) made for example of an a-Si TFT and a pixel electrode 108connected to this switching element are formed on the array substrate106.

Then, as shown in FIG. 18(b), the left region of the pixel electrode 108is irradiated with UV light under an ozone atmosphere, and is madeflatter than the right region of the pixel electrode 108, thus forming aflattened region 108 a.

Then, as shown in FIG. 18(c), a pre-imide polyimide alignment materialby JSR Corp. is spread on the pixel electrode 108, and dried and baked,to form the alignment film 110.

In this manner, the pretilt angle of the liquid crystal molecules 140arranged on the flattened region 108 a of the pixel electrode 108 can bemade smaller than the pretilt angle of the liquid crystal molecules 140arranged on the non-flattened region 108 b. Furthermore, by performingthe same process with the common electrode as well, a liquid crystaldisplay device is attained that has the first liquid crystal cell regionand the second liquid crystal cell region shown in FIG. 16 within thesame pixel.

Then, as shown in FIG. 16, the surface of the alignment film 109 and ofthe alignment film 110 formed as above and provided with small and largepretilt angles in opposition to one another, is subjected to a parallelalignment process by rubbing with a rubbing cloth in a directionperpendicular to the signal electrode lines 113 and in the samedirection for the upper and the lower substrate (that is, from left toright in FIG. 16), arranging the liquid crystal layer 112 of positivenematic liquid crystal material.

In the liquid crystal display device manufactured in this manner, thesmaller pretilt angle A2 is arranged toward the alignment origin (nearthe source of the rubbing movement) of the pixel electrode 108, and thelarger pretilt angle C2 is arranged in opposition to the pretilt angleA2. Applying a first voltage of 2.5V between the common electrode 107and the pixel electrode 108, a b-splay alignment 120, in which theliquid crystal molecules are splay aligned on the side of the arraysubstrate 106, forms easily in the region (I) of the pixel in FIG. 16(first liquid crystal cell region), and a t-splay alignment 121, inwhich the liquid crystal molecules are splay aligned on the side of theopposing substrate 105, forms easily in the region (II) of the pixel(second liquid crystal cell region).

As shown in FIGS. 16 and 17, when applying a first voltage of 2.5Vbetween the common electrode 107 and the pixel electrode 108 through theswitching element 111 of the liquid crystal cell 104, a b-splayalignment region (first liquid crystal region) and a t-splay alignmentregion (second liquid crystal region) are formed in the pixel, and atthe border between the two, a disclination line 123 is clearly formedalong the signal electrode line 113 and straddling the gate electrodelines 114 and 114′ (disclination line forming step).

Repeatedly applying a −15V voltage pulse as a second voltage between thecommon electrode 107 and the pixel electrode 108, transition seeds werecreated starting at the disclination line 123 as shown in FIG. 17, andspreading the transition to bend alignment 124, all of the TFT panelpixels transitioned swiftly in three seconds (alignment transitionstep).

It seems that in the disclination line region at the border betweenregions of b-splay alignment and t-splay alignment, the bending energyis higher than in the surrounding regions, and when in this state, evenmore energy is directed to it by applying a high voltage to the upperand lower electrodes, the splay alignment transitions into bendalignment.

Eighth Embodiment

FIG. 19 shows a diagrammatic drawing of a liquid crystal display devicein accordance with the eighth embodiment of the present invention.

During regular display, the gate electrode lines are scanned line byline and turned on, but before regular display, the gate electrode linesare turned on one by one, and repeatedly applying a −15V voltage pulseas a second voltage between the common electrode 107 and the pixelelectrode 108 generates a transversal electric field caused by thepotential difference between the pixel electrode 108 and the gateelectrodes 114 and 114′. Then, due to this transversal field, transitionseeds appear starting near the disclination line 123 and the gateelectrode lines 114 and 114′ as shown in FIG. 19, spreading thetransition into bend alignment, and all of the TFT panel pixelstransition swiftly within about one second (alignment transition step).

It seems that in the disclination line region at the border betweenregions of b-splay alignment and t-splay alignment, the bending energyis higher than in the surrounding regions, and directing even moreenergy to the disclination line by applying a transversal field from thetransversally arranged gate electrode lines in this state leads to aswift transition. After the transition is finished, the gate electrodelines 114 and 114′ are returned to their regular scanning state.

It should be noted that it is also possible to apply the second voltagebetween the pixel electrode and the common electrode continuously. Also,the effect of speeding up the transition can be attained when apulse-shaped voltage is applied repeatedly, if its frequency is in arange of 0.1 Hz to 100 Hz, and the duty ratio of the second voltage isat least 1:1 to 1000:1.

Other Considerations

In the seventh and eighth embodiments, the pretilt angle D2 in thepre-alignment region of the common electrode that was aligned first wasset to a smaller value, but it can also be set to a larger value.Moreover, the pretilt angle B2 in the region of the pixel electrode thatwas aligned first was set to a larger value, but an effect can also beattained with a small value, because t-splay alignment is assumed due tothe influence of the transversal electric field.

Moreover, the pretilt angle A2 was set to 2° on one substrate and thepretilt angle C2 was set to 5° on the opposing substrate, and makingthis ratio is larger has the effect of shortening the transition time,so that the transition time can be made even faster.

Moreover, in the preceding embodiments, the value of the smaller pretiltangle A2 was set to 2°. In order to achieve an easy transition fromb-splay alignment to bend alignment, it is sufficient if the smallerpretilt angles A2 and D2 are not larger than 3°, and the larger pretiltangles B2 and C2 are at least 4°.

Moreover, the upper and lower substrates have been subjected to aparallel alignment process in which the alignment processing directionwas in the same direction perpendicular to the signal electrode lines113, but it is also possible to perform a parallel alignment process inwhich the alignment processing direction is the same directionperpendicular to the gate electrode lines 114 (that is, in a directionperpendicular to the paper plane of FIG. 16). In that case, thedisclination line is formed at a different location.

Moreover, if the alignment processing direction forms an angle of forexample about 2° with the direction perpendicular to the electrode linesalong the pixel electrodes, then the transversal electric field isapplied at an angle from the electrodes to the disclination line in thepixel, so that the force twisting the splay aligned liquid crystalmolecules increases, assisting the transition into the bend alignment,leading to a liquid crystal display device with a reliable and swifttransition.

For the first voltage, a voltage is sufficient that is equal to orhigher than the voltage at which the disclination line can be formed.Also, a second voltage was applied between the pixel electrode and thecommon electrode, but the second voltage can also be applied to thecommon electrode.

Moreover, a polyimide material was used as the material for thealignment film, but it is also possible to use another material, such asa monomolecular layer.

It is also possible to make the substrates in the liquid crystal displaydevice of plastic, for example. Furthermore, it is also possible to usea reflecting substrate, such as silicon, for one substrate.

Ninth Embodiment

In this embodiment, intermitting protrusions and recesses are formedwith the signal electrode lines and pixel electrodes, as well as withthe gate electrode lines and the pixel electrodes.

FIGS. 20 and 21 diagrammatically illustrate the main elements of aliquid crystal display device of this embodiment.

These drawings show a pixel of an active matrix OCB-mode liquid crystaldisplay device, taken from above the display (on the user's side).

In FIG. 20, numeral 206 denotes signal electrode lines (bus lines),numeral 207 denotes gate electrode lines, and numeral 208 denotes aswitching transistor (element).

In these drawings, the signal electrode lines 206 intersect with thegate electrode lines 207, but needless to say, they are arranged as skewlines with an insulating film (not shown in the drawings) interposedbetween them.

Moreover, in these drawings, the switching transistor 208, which is aTFT, is connected to a substantially square pixel electrode 202 a.Function, operation and effect of the signal electrode lines 206, thegate electrode lines 207, the switching transistor 208, and the pixelelectrode 202 a are as in a OBC-mode or other conventional liquidcrystal display device.

And as in conventional liquid crystal display devices, the upper andlower alignment films 203 a and 203 b are subjected to an alignmentprocess using, for example, a rubbing cloth to put the liquid crystalmolecules 211 initially into splay alignment.

Moreover, as in conventional liquid crystal display devices, light anddark display is achieved by the effect of the polarizers 204 a and 204b, and the effect of the transition of all the liquid crystal moleculesin the pixels between the opposing substrates from splay alignment tobend alignment.

However, as shown in FIG. 20(a), recesses 221 a and protrusions 222 aare formed approximately at the center of the sides of the approximatelysquare pixel electrode 202 a. Complementary to that, the adjacent signalelectrode lines 206 and the gate electrode lines 207 are deformed andprovided with protrusions 261 and 271 and recesses 262 and 272 fittinginto the recesses 221 a and protrusions 222 a. Thus, different fromconventional liquid crystal display devices, the upper and lower sidesand the left and right sides (with respect to the paper plane of FIG.20(a)) of the pixel electrodes 202 a are provided with deformedtransition-inducing transversal electric field application portions.

The following is an explanation of a method for manufacturing thisliquid crystal display device.

A polyimide alignment film material of polyamic acid type (by NissanChemical Industries, Ltd.), having a large pretilt angle of about 5° isspread on the surface of the pixel electrode 202 a with the transversalelectric field application portions and the common electrode 202 b,dried and baked, thereby forming the alignment films 203 a and 203 b onthese electrode surfaces on the side of the liquid crystal layer 210.

Then, the surfaces of the alignment films 203 a and 203 b are bothsubjected to an alignment process by rubbing with a rubbing cloth in adirection substantially perpendicular to the signal electrode lines 206,as shown in FIG. 20(a).

Then, a positive nematic liquid crystal material was vacuum injectedbetween the upper and the lower substrate, forming the liquid crystallayer 210.

Thus, although this is not shown in the drawings, on the surface of theupper and lower alignment films 203 a and 203 b, the pretilt angles ofthe liquid crystal molecules 211 have opposite signs, and the moleculesare aligned with the long axis direction substantially in parallel, sothat when the liquid crystal layer 210 is in the so-called non-voltageapplication state, the liquid crystal molecules are spread obliquely inso-called splay alignment.

The following explains how display with this liquid crystal displaydevice works.

A pulse-shaped voltage of −15V, which is a relatively high voltage inthe field of liquid crystals, is applied repeatedly between the commonelectrode 202 b and the pixel electrode 202 a, and the gate electrodelines 207 are either put into the regular scanning state or almost allgate electrode lines 207 are put into the ON state. Due to thetransversal electric field application portions, a transversal electricfield, that is larger than the usual transversal electric field aroundit, is applied between the gate electrode lines 207, the signalelectrode lines 206 and the pixel electrode 202 a. As a result, if thesplay alignment region within the pixel region has been rubbed in adirection that is substantially perpendicular to the signal electrodelines 206, transition seeds for a transition toward bend alignment inthe liquid crystal layer 299 appear, starting mainly at the transversalelectric field application portions between the gate electrode lines 207and the pixel electrode 202 a. Moreover, if the splay alignment regionhas been rubbed in a direction that is perpendicular to the gateelectrode lines 207, transition seeds for a transition toward bendalignment in the liquid crystal layer 298 appear, starting mainly at thetransversal electric field application portions between the signalelectrode lines 206 and the pixel electrode 202 a, as shown in FIG. 21.

Furthermore, as a result of the spreading of the bend alignment regionaround these transition seeds, transition into the bend alignment couldbe accomplished in about 0.5 s for the entire pixel region.

The entire TFT panel transitioned swiftly within about 2 s.

It seems that with this configuration, applying a high voltage betweenthe upper and the lower electrodes, the liquid crystal layer 210 assumesb-splay alignment as shown in FIG. 20(b), the bending energy becomeshigher than in the surrounding areas, and since the transversal electricfield application portions apply a transversal field that issubstantially perpendicularly (that is, perpendicularly to the plane ofFIG. 20(b)) to the alignment of the liquid crystal molecules in thisstate, the liquid crystal molecules on the lower substrate side inb-splay alignment in FIG. 20(b) are twisted, leading to the creation oftransition seeds.

In the foregoing explanation, the transversal electric field applicationportions were formed such that that the recesses and protrusions in thedeformed pixel electrode fitted into the recesses and protrusions of thesignal electrode lines, but as shown in FIG. 22, it is of course alsopossible to form the transversal electric field application portionsonly in the pixel electrode 202 a, only in the signal electrode lines206, or only in the gate electrode lines 207.

That is to say, the difference between in FIG. 22 and FIG. 20 is that inFIG. 22, the protrusions do not fit into the recesses, and there is onlyone of the protrusion 263 in the signal electrode line 206, theprotrusion 273 in the gate electrode line 207, and the protrusions 223 aand 224 a in the pixel electrode 202 a.

Furthermore, the planar shape of the protrusions and recesses can betriangular or rectangular as shown in FIGS. 20 and 22, but it can ofcourse also be trapezoid, semi-circular, circular, or elliptical, forexample.

Moreover, in FIGS. 20 and 22, the transversal electric field applicationportions are provided at a total of four locations, namely at top andbottom as well as left and right, but in accordance with for example thesize of the pixel, it is also possible to provide them only at twolocations or even only one location, and needless to say, it is alsopossible to form protrusions and recesses continuously along theelectrode edges. Also, in the preceding, the rubbing direction wassubstantially perpendicular to either the signal electrode line or thegate electrode line, but the rubbing direction can also be diagonal. Inthat case, the transition into bend alignment will start at the liquidcrystal layer at the transversal electric field application portionsbetween the signal and gate electrode lines and the pixel electrode. Itis also preferable that each pixel is provided with at least onetransversal electric field application portion that can apply atransversal electric field in at least a direction substantiallyperpendicular to the rubbing direction.

Since FIGS. 20 and 22 are plan views, both types of electrode lines(signal electrode lines 206 and gate electrode lines 207) and the pixelelectrode 202 a appear to be in the same plane, but at least one type ofelectrode lines can be arranged at a different height from the pixelelectrode on the array substrate.

Thus, the transversal electric field application portions made ofelectrode deformations, in which a portion of the sides of the pixelelectrode is deformed to protrusions and recesses within a plane that isparallel to the substrate plane, are separated by about 0.5 to 10 μm invertical direction, and the transversal electric field is generated dueto the presence of the protrusions of the signal electrode lines or gateelectrode lines on the sides of the transversal electric fieldapplication portions and the recesses with dents of about 0.5 to 10 μm.

Tenth Embodiment

In this embodiment, electrode lines are provided for application of thetransversal electric field.

The following explains this embodiment with reference to FIG. 23.

FIG. 23(a) is a plan view taken from above the substrate. FIG. 23(b) isa cross-sectional view taken in parallel to the gate electrode lines 207of the liquid crystal display device.

In FIGS. 23(a) and 23(b), numeral 209 denotes electrode lines dedicatedto application of a transversal electric field and arrangedapproximately directly below the signal electrode lines 206 on the arraysubstrate 201 a. Numeral 212 denotes a transparent insulating film forinsulating the transversal electric field application lines 209 from thesignal electrode lines 206 and the gate electrode lines 207.Consequently, when viewing this pixel from above (that is, from a user'sviewing direction perpendicular to the display screen), protrusions 291of triangular shape (when viewed from above) project from thetransversal electric field application lines 209 toward the signalelectrode lines 206 at the center on the left and right of the pixel, asshown in FIG. 23(a). It should be noted that the signal electrode lines206 and the pixel electrode 202 a are the same as in the prior art.

The transversal electric field application lines 209 are connected to adriving circuit connected to the signal electrode lines 206 or the gateelectrode lines 207, and during regular liquid crystal display after thealignment transition, the transversal electric field application lines209 are disconnected from the driving circuit.

It is also possible to arrange the transversal electric fieldapplication lines 209 as signal electrode lines above the signalelectrode lines 206, near the pixel electrodes and separated from thepixel electrodes by a transparent insulating film, increasing the effectof the transversal field application, and to electrically connect themcollectively by contact holes (not shown in the drawings) in thetransparent insulating film. In that case, the redundancy is increasedas there are two signal electrode lines, and there is also the effect ofreduced electrical resistance.

That is to say, the transversal electric field application lines 209 aare provided directly above the signal electrode lines 206, separated bythe transparent insulating film 213, as shown in FIG. 23(c). Also inthis case, protrusions 291 a that are triangular when viewed from aboveprotrude toward the pixel center.

FIG. 23(d) shows another example of this embodiment. As shown in FIG.23(d), the transversal electric field application lines 209 b arecovered by a transparent flattening insulating film 212 b, below thededicated lines 209 b, the signal electrode lines 206 are covered by atransparent flattening insulating film 212 c, and the pixel electrode202 a is arranged on the transparent flattening insulating film 212 b.Also in this case, protrusions 291 b that are triangular when viewedfrom above protrude toward the pixel center.

In the drawings, the protrusions of the dedicated lines for transversalelectric field application are triangular, but it is of course alsopossible to provide the protrusions continuously along the entireportion in opposition to the pixel electrodes, to provide protrusionsthat have a three-dimensional structure, projecting upward for example.

It is also possible to provide the dedicated lines for transversalelectric field application not directly below the signal electrodelines, but directly below the gate electrode lines. Furthermore, theycan also be provided directly below both types of electrode lines.

Eleventh Embodiment

In this embodiment, defect portions are formed, in which cutouts areprovided at least at one location in the pixel electrodes.

FIG. 24 diagrammatically shows a plan view of one pixel unit in a liquidcrystal display device in accordance with this embodiment. As shown inFIG. 24, a portion of several μm width is etched away from the pixelelectrode 202 a made of an ITO film, forming a electrode defect portion225 that is crank-shaped when viewed from above.

A polyimide alignment film material of polyamic acid type (by NissanChemical Industries, Ltd.), having a pretilt angle of about 5° is spreadon the surface of the pixel electrode 202 a including this electrodedefect portion 225 and on the surface of the common electrode (not shownin the drawings), dried and baked, thereby forming alignment films (notshown in the drawings). Then, the surfaces of these alignment films areboth subjected to an alignment process by rubbing with a rubbing clothin a direction perpendicular to the gate electrode lines 207, so thatthe pretilt angles of the liquid crystal molecules have opposite signs,and the molecules are aligned substantially in parallel, which is thesame as in the ninth and tenth embodiments.

Consequently, the liquid crystal layer also has so-called liquid crystalcells in splay alignment, made of regions in which the liquid crystalmolecules widen up diagonally when no voltage is applied.

However, if, before display, a voltage pulse of 15V is appliedrepeatedly between the common electrode and the pixel electrode of thepixel, or a voltage pulse of −15V is applied repeatedly to the commonelectrode, and the gate electrodes are either put into the regularscanning state or almost all gate electrodes are put into the ON state,then, due to the electrode cutout portions 225 in each pixel unit, astrongly bent diagonal transverse electric field 280 is generated at theedge of the electrode cutout portions 225, as shown in FIG. 24(b).

Therefore, seeds for transition into the bend alignment appear in theliquid crystal layer 299 of these electrode cutout portions 225, thesebend alignment regions spread, and the splay alignment in the pixelregion transitions completely into bend alignment in about 0.5 s for theentire pixel region. The entire TFT panel transitions swiftly withinabout 2 s.

It seems that as a strong transversal electric field is generated at thetransversal electric field application portions made of these electrodecutout portions 225 and the liquid crystal molecules nearby are alignedhorizontally in the substrate plane in a so-called b-splay alignment,the bending energy becomes higher than in the surrounding areas, and asa result of applying even more energy with the high voltage appliedbetween the upper and lower electrodes in this situation, transitionseeds appear at the electrode defect portions 225, and the bendalignment regions spread.

Only one electrode defect portion 225 that is crank-shaped when viewedfrom above is formed in FIG. 24, but it is of course also possible toform two or more.

Also, its form can of course also be straight, angular, circular,elliptical, or triangular, for example.

Furthermore, the electrode cutout portions 225 can also be formed on theside of the common electrode.

Needless to say, they can also be formed on both the pixel electrode andthe common electrode.

Twelfth Embodiment

In this embodiment, a transverse electric field is generated, andregions with different tilt angles in the pixel plane are formedaccordingly.

FIG. 25 diagrammatically illustrates the configuration and features ofone pixel unit in a liquid crystal display device of this embodiment.FIG. 25(a) is a cross-sectional view of the pixel, taken in parallel tothe gate electrode lines, and shows how the tilt angles are differentwithin the same pixel in region (I) on the left side and region (II) onthe right side.

FIG. 25(b) is a plan view of the pixel taken from above (where the useris). Recesses and protrusions 221 a and 222 a are provided at top andbottom as well as left and right of the pixel electrode 202 a, andrecesses and protrusions 261, 262, 271 and 272 fitting into the recessesand protrusions 221 a and 222 a are provided at corresponding positionsof the signal electrode lines 206 and the gate electrode lines 207. Asin the seventh embodiment explained above, a disclination line 226 isformed at the border between the regions (I) and (II) in FIG. 25 whenapplying a first voltage of 2.5V.

The following is an explanation of a method for manufacturing the liquidcrystal display device of this embodiment.

Alignment films 203 am and 203 bm are formed on the inner faces ofopposing substrates for active matrix liquid crystal cells. Thesealignment films 203 am and 203 bm are subjected to a process forming asplay alignment when no voltage is applied to the liquid crystal layer210, and transition-inducing transversal electric field applicationportions are formed for example at the pixel electrodes 202 a and thegate electrode lines 207 arranged nearby, which is the same as in thefirst embodiment.

However, the processing of the alignment films is different. That is tosay, in FIG. 25(a), a polyimide alignment film material of polyamic acidtype (by Nissan Chemical Industries, Ltd.), having a large pretilt angleB2 of about 5° is spread on the surface of the pixel electrode 202 awith the transversal electric field application portions, dried andbaked, thereby forming the alignment film 203 am.

Then, only the left region 203 ah of the alignment film 203 am, that is,only the region (I) is irradiated with UV light to decrease the pretiltangle E2 to a smaller angle of about 2°.

On the other hand, a polyimide alignment film material of polyamic acidtype (by Nissan Chemical Industries, Ltd.), providing the boundaryliquid crystal molecules with a large pretilt angle F2 of about 5° isspread on the opposing substrate 201 b, dried and baked, thereby formingthe alignment film 203 bh on the common electrode 202 b.

Then, only the right region 203 bm of the alignment film 203 bh, thatis, only the region (II) is irradiated with UV light to decrease thepretilt angle D2 to a smaller angle of about 2°.

Thus, a smaller pretilt angle E2 is formed in the alignment film 203 ahon the left half of the array substrate 201 a in opposition to a largerpretilt angle F2 in the alignment film 203 bh in the left half of theopposing substrate 201 b as shown in region (I) in FIG. 25(a), and alarger pretilt angle B2 is formed in the alignment film 203 am in theright half of the array substrate 201 a in opposition to a smallerpretilt angle D2 in the alignment film 203 bm in the right half of theopposing substrate 201 b as shown in region (II).

Moreover, the surfaces of the alignment films, provided with small andlarge pretilt angles arranged like this in opposition to one another,are both subjected to a parallel alignment process by rubbing with arubbing cloth in a direction substantially perpendicular to the signalelectrode lines 6 in the same direction for the upper and the lowersubstrate, as shown in FIG. 25(b). Then, a positive nematic liquidcrystal material is filled in, forming the liquid crystal layer 210.

With this configuration, the smaller pretilt angle E2 is arranged towardthe alignment origin (toward the source of the rubbing movement) in thepixel electrode 202 a, and the larger pretilt angle F2 is arranged inopposition to this pretilt angle E2, and a b-splay alignment 227 b, inwhich the liquid crystal molecules are splay aligned at the lowersubstrate forms more readily in the pixel region (I) in FIG. 25(a), anda t-splay alignment 227 t, in which the liquid crystal molecules aresplay aligned at the upper substrate forms more readily in the pixelregion (II).

Then, applying a voltage of 2.5V, which is near the transition criticalvoltage, to the opposing electrodes through the switching transistors208 of the liquid crystal cells, a b-splay alignment region and at-splay alignment region were formed in the same pixel (for the reasonsexplained above), and a discination line 226 was formed clearly at theborder of these alignment regions along the signal electrode lines 206,straddling the gate electrode lines 207.

Repeatedly applying a −15V voltage pulse between the common electrodeand the pixel electrode in this pixel, transition seeds appearedstarting at the liquid crystal layer 299 near the disclination line 226and near the transversal electric field application portions as shown inFIG. 25(b), the transition to bend alignment regions spread, and all ofthe TFT panel pixels transitioned swiftly within about one second.

It seems that in the region of the disclination line 226 at the borderbetween regions of b-splay alignment and t-splay alignment, the bendingenergy is higher than in the surrounding regions, and due to thetransversal electric field generated at the transversal fieldapplication portions in addition to this state, the splay alignment istwisted and transitions more readily, and applying a high voltagebetween the upper and lower electrodes, even more energy is directed toit, leading to the transition into bend alignment.

Above, several embodiments of the present invention have been explained,but it is to be understood that these embodiments are in no way intendedto limit the present invention. For example, the following variationsare possible:

1) The voltage is applied between pixel electrode and common electrodeis continuous or an intermittent.

2) When the high voltage pulse is applied repeatedly, its frequency isin the range from 0.1 Hz to 100 Hz, and the duty ratio of the secondvoltage is in the range of at least 1:1 to 1000:1, selecting values thataccelerate the transition.

3) Plastic is used for the substrates, and organic conducting films areused for the electrodes.

4) One of the substrates is a reflective substrate, having reflectingelectrodes of silicon or aluminum for example, for a reflective liquidcrystal display device.

5) As an additional measure, the pixel electrodes and the commonelectrode are provided with protrusions for the generation of a strongelectrode electric field in a direction perpendicular to the substrateplane.

6) As an additional measure, protrusions instead of globular glass orsilica beads are provided to keep a predetermined distance between thesubstrates, and these protrusions also have the function to arrange theliquid crystal molecules.

7) The upper portions or the lower portions of these protrusions alsoserve as the protrusions for strong electrode generation.

8) The shape of the pixel electrodes is oblong or triangular, instead ofsquare.

9) The pixels are partitioned not into two regions but into three orfour regions with different liquid crystal alignments.

10) As a measure for imparting small and large pretilt angles, thesurface constitution of the transparent electrodes is changed by O₂ashing, and alignment films are formed on these transparent electrodes.

Thirteenth Embodiment

FIG. 26 shows a diagram of the configuration of a test cell used to testthe splay—bend transition time in a liquid crystal display device of thepresent invention. FIGS. 27 and 28 show parts of the manufacturingprocess and illustrate how the bump-shaped protrusions are made.

A PC resist material (by JSR Corp.) is spread on a glass substrate 308,forming a resist thin film of 1 μm thickness. Then, collimated UV light323 is irradiated through a photo mask 321 with rectangular apertures322 to expose the resist thin film 320. The resist thin film 320 thathas been exposed with the collimated light is developed, rinsed, andpre-baked at 90° C., forming bump-shaped protrusions 310 with across-section as shown in FIG. 28.

Then, ITO electrodes 7 of 2000 A thickness are formed on this substratewith one of the usual methods, forming a glass substrate 308 withelectrodes. Subsequently, an alignment film coating (SE-7492 by NissanChemical Industries, Ltd.) is spread by spin-coating on a glasssubstrate 301 with transparent electrodes 302 and on the glass substrate308 provided with the bump-shaped protrusions, and cured for one hour ina thermostatic bath at 180° C., thus forming the alignment films 303 and306. Then, a rubbing process is carried out, rubbing with a rubbingcloth by Toho Rayon Co., Ltd. in the direction shown in FIG. 29, andusing spacers 5 by Sekisui Fine Chemical Corp. and Struct Bond 352A(tradename for a sealing resin by Mitsui Toatsu Chemicals, Inc.), thesubstrates were laminated together at a spacing of 6.5 μm, thusproducing a liquid crystal cell 309 (referred to as “liquid crystal cellA” in the following).

The rubbing process was carried out such that the pretilt angle of theliquid crystal was about 5° at the boundaries to the alignment film.

Then, a liquid crystal MJ96435 (with a refractive index anisotropy ofΔn=0.138) was vacuum injected into the liquid crystal cell A, thusyielding the test cell A.

Then, polarizers were laminated on the test cell A, such that thepolarization axes of the polarizers formed an angle of 45° with thedirection of the rubbing process, and the polarization axes crossed atright angles. Applying a 7V square voltage pattern and observing thetransition from splay alignment to bend alignment, the transition fromsplay alignment to bend alignment took about 5 s for the entireelectrode region.

In the regions where the bump-shaped protrusions 310 are formed, theliquid crystal layer is thinner than in the surrounding regions,effectively increasing the strength of the electric field, so that areliable bend transition starting at these portions can be ensured. Thebend alignment then spreads swiftly to the other regions.

That is to say, a reliable and fast splay→bend transition can beachieved.

The cross-section of the bump-shaped protrusions can of course also betrapezoid, triangular, or semi-circular, instead of rectangular as inthe present embodiment.

As a comparative example, a splay alignment liquid crystal cell R wasmanufactured with the same process as above, except that aglass-substrate with transparent electrodes, but without the bump-shapedprotrusions 310 was used, into which liquid crystal MJ96435 was filled,thus yielding a test cell R. Applying a 7V square voltage pattern to thetest cell R, the time needed for the transition from splay alignment tobend alignment for the entire electrode region was 42 s, which clearlydemonstrates the effect that the present invention has.

Fourteenth Embodiment

FIG. 30 shows a diagram of the configuration of a test cell used to testthe splay—bend transition time in a liquid crystal display device of thepresent invention, and FIG. 31 is a plan view thereof. FIG. 30 is across-sectional view taken from the direction of the arrows X1—X1 inFIG. 31. The fourteenth embodiment is characterized in that thebump-shaped protrusions 310 are provided on a transparent electrode 307a, which is formed outside the display pixel region. The followingexplains a procedure for manufacturing the same.

An alignment film coating (SE-7492 by Nissan Chemical Industries, Ltd.)is spread by spin-coating on the glass substrate 301 with transparentelectrodes 302 and on the glass substrate 308 provided with thebump-shaped protrusions, and cured for one hour in a thermostatic bathat 180° C., thus forming the alignment films 303, 306 and 306 a. Then, arubbing process is carried out, rubbing with a rubbing cloth by TohoRayon Co., Ltd. in the direction shown in FIG. 29, and using spacers 305by Sekisui Fine Chemical Corp. and Struct Bond 352A (tradename for asealing resin by Mitsui Toatsu Chemicals, Inc.), the substrates werelaminated together at a spacing of 6.5 μm, thus producing a liquidcrystal cell (referred to as “liquid crystal cell B” in the following).The rubbing process was carried out such that the pretilt angle of theliquid crystal was about 5° at the boundaries to the alignment films.

Then, a liquid crystal MJ96435 (with a refractive index anisotropy ofΔn=0.138) was vacuum injected into the liquid crystal cell B. Then,polarizers were laminated on the test cell B, such that the polarizationaxes of the polarizers formed an angle of 45° with the direction of therubbing process for the alignment films, and the polarization axescrossed at right angles. Applying a 7V square voltage pattern andobserving the transition from splay alignment to bend alignment, thetransition from splay alignment to bend alignment took about 7 s for theentire electrode region.

In this embodiment, the bumps-shaped protrusions are provided outsidethe display pixel regions, and the bend transition seeds appear outsidethe display pixel regions, but it could be confirmed that the bendalignment spreads swiftly from outside the display pixel regions intothe display pixel regions.

There is a region in which no electric field is applied between thedisplay pixel region and the electrode region for the creation of bendseeds (region without electrode portion), but if this region is small,then the bend alignment can expand beyond this region.

Fifteenth Embodiment

FIG. 32 is a diagram of the configuration of a test cell used to testthe splay—bend transition time in a liquid crystal display device of thepresent invention. FIGS. 27, 28 and 33 show parts of the manufacturingprocess and illustrate how the bump-shaped protrusions are made.

A PC resist material (by JSR Corp.) is spread on a glass substrate 308,forming a resist thin film of 1 μm thickness. Then, collimated UV light323 is irradiated through a photo mask 321 with rectangular apertures322 to expose the resist thin film 20. The resist thin film 20 that hasbeen exposed with the collimated light is exposed, rinsed, and pre-bakedat 90° C., forming bump-shaped protrusions 310 with a cross-section asshown in FIG. 28.

Then, the resist thin film 320 is post-baked at 150° C., which is abovethe glass transition point of the resist thin film material, to slopethe shoulders of the bump-shaped protrusions 310 gently downward, andthe bump-shaped protrusions 310 are provided with a triangularcross-section as shown in FIG. 32.

Then, ITO electrodes of 2000 A thickness are formed on this substratewith one of the usual methods, forming a glass substrate 308 withelectrodes. Subsequently, an alignment film coating (SE-7492 by NissanChemical Industries, Ltd.) is spread by spin-coating on the glasssubstrate 301 with the transparent electrodes 302 and on the glasssubstrate 308 provided with the bump-shaped protrusions, and cured forone hour in a thermostatic bath at 180° C., thus forming the alignmentfilms 303 and 306. Then, a rubbing process is carried out by rubbingwith a rubbing cloth by Toho Rayon Co., Ltd. in the direction shown inFIG. 29, and using spacers 305 by Sekisui Fine Chemical Corp. and StructBond 352A (tradename for a sealing resin by Mitsui Toatsu Chemicals,Inc.), the substrates were laminated together at a spacing of 6.5 μm,thus producing a liquid crystal cell 309 (referred to as “liquid crystalcell C” in the following).

The rubbing process was carried out such that the pretilt angle of theliquid crystal was about 5° at the boundary to the alignment film.

Then, a liquid crystal MJ96435 (with a refractive index anisotropy ofΔn=0.138) was vacuum injected into the liquid crystal cell C, thusyielding the test cell C.

Then, polarizers were laminated on the test cell C, such that thepolarization axes of the polarizers formed an angle of 45° with thedirection of the rubbing process, and the polarization axes crossed atright angles. Applying a 7V square voltage pattern and observing thetransition from splay alignment to bend alignment, the transition fromsplay alignment to bend alignment took about 7 s for the entireelectrode region.

In this test cell C, concentrations of the electric field occur at thetriangular tips, and bend alignment starts from these portions.Moreover, above the triangular portions 60, there are portions that arerubbed downward and portions that are rubbed upward in the rubbingprocess, so that as a result, regions are formed in which the liquidcrystal pretilt angle has the opposite sign. That is to say, near thebump-shaped protrusions, the liquid crystal directors are horizontal inthe substrate plane, and this seems to contribute to a fast splay—bendtransition.

In this embodiment, the electric field concentration portions areprovided within the pixel regions, but a similar effect could beconfirmed when providing them outside the pixel regions. Also, in thisembodiment, the electric field concentration portions were provided onlyon one of the two substrate sides, but it is also possible to providethem on both substrate sides.

Sixteenth Embodiment

FIG. 34 is a diagram of the configuration of a test cell used to testthe splay—bend transition time in a liquid crystal display device of thepresent invention. FIG. 35 illustrates the electrode pattern of theglass substrate 302 used in this example.

An alignment film coating (SE-7492 by Nissan Chemical Industries, Ltd.)is spread by spin-coating on and on a glass substrate 301 provided witha transparent electrode 302 having an aperture portion 380 and on aglass substrate 308 provided with a transparent electrode 307 withoutaperture portion, and cured for one hour in a thermostatic bath at 180°C., thus forming the alignment films 303 and 306. Then, a rubbingprocess is carried out, rubbing with a rubbing cloth by Toho Rayon Co.,Ltd. in the direction shown in FIG. 29, and using spacers 305 by SekisuiFine Chemical Corp. and Struct Bond 352A (tradename for a sealing resinby Mitsui Toatsu Chemicals, Inc.), the substrates were laminatedtogether at a spacing of 6.5 μm, thus yielding a liquid crystal cell 309(referred to as “liquid crystal cell D” in the following).

The rubbing process was carried out such that the pretilt angle of theliquid crystal was about 5° at the boundaries to the alignment films.

Then, a liquid crystal MJ96435 (with a refractive index anisotropy ofΔn=0.138) was vacuum injected into the liquid crystal cell D, thusyielding the test cell D.

Then, polarizers were laminated on the test cell D, such that thepolarization axes of the polarizers formed an angle of 45° with thedirection of the rubbing process for the alignment films, and thepolarization axes crossed at right angles. Applying a voltage, thetransition from splay alignment to bend alignment was observed.

In this test cell D, when a square voltage of 2V and 30 Hz was appliedto the electrode on the side of the glass substrate 8, and a squarevoltage of 7V and 30 Hz was applied to the electrode on the side of theglass substrate 1, the transition from splay alignment to bend alignmenttook about 5 s for the entire electrode region, so that a very fast bendtransition could be achieved.

In this embodiment, an electric field of 5V (=7V−2V) is applied to theliquid crystal layer disposed between the two electrodes, but since aneffective field of 7V (=7V−0V) is applied to the liquid crystal layer atthe electrode aperture portion, bend alignment occurs at that portion.

In this embodiment, the aperture portion was rectangular, but of courseit can also have another shape, such as circular or triangular.

Seventeenth Embodiment

FIG. 36 is a cross-sectional view showing the main portions of a liquidcrystal display element in accordance with the seventeenth embodiment.FIG. 37 is a magnification of a portion of FIG. 36. This liquid crystaldisplay device has pixel switching elements 380, signal electrode lines381, and gate signal lines (not shown in the drawings) formed on theglass substrate 308, and flattening films 382 cover the switchingelements 380, the signal electrode lines 381 and the gate signal lines.Display electrodes 307 are formed on the flattening films 382, and thedisplay electrodes 307 and the switching elements 380 are electricallyconnected by relay electrodes 384 passing through contact holes 383 inthe flattening films 382. In the portions on the upper aperture side ofthe contact holes 383, the relay electrodes 384 are provided withrecesses 384 a, as shown in FIG. 37. These recesses 384 a form aperturesin the display electrodes 307, and electric field concentrations can beattained near these recesses 384 a. Consequently, a shortening of thetransition time can be achieved.

Eighteenth Embodiment

FIG. 38 diagrammatically shows the configuration of a liquid crystaldisplay element in accordance with the present invention.

Phase difference plates 312 and 315 made of an optical medium withnegative refractive index anisotropy whose main axes are in hybridarrangement, negative uniaxial phase difference plates 311 and 314, apositive uniaxial phase difference plate 319, and polarizers 313 and 316are laminated in the configuration shown in FIG. 39 onto the test cell Dmade in the third embodiment, thus yielding a liquid crystal displayelement D.

The retardation of the phase difference plates 312, 315, 311, 314, and319 is 26 nm, 26 nm, 350 nm, 350 nm, and 150 nm, respectively, for lightof 550 nm wavelength.

FIG. 40 shows the voltage—transmissivity characteristics for the frontface of the liquid crystal display element D at 25° C. The measurementwas performed while lowering the voltage after applying a 10V squarevoltage for 10 s and confirming bend alignment. In this liquid crystaldisplay element, the transition from bend alignment to splay alignmentoccurs at 2.1V, so that display has to be performed effectively at avoltage of at least 2.2V.

Then, it was measured how the contrast ratio depends on the viewingangle for a white level voltage of 2.2V and a black level voltage of7.2V, and it was determined that a contrast ratio of at least 10:1 isattained over a range of 126° vertical and 160° horizontal. It was alsoconfirmed that a sufficiently broad viewing angle could be maintainedwhen providing portions in which the orientation of the liquid crystaldirectors on the substrate alignment films differs from surroundingportions. Furthermore, alignment defects and defects in the displayquality could not be observed with the bare eye.

When the response time for 3V to 5V was measured, the rise time was 5ms, and the fall time was 6 ms.

As becomes clear from the above, with the liquid crystal display deviceof the present invention, a fast splay—bend alignment transition can beachieved without sacrificing the broad viewing angle characteristics orthe response characteristics of the conventional OCB-mode, which is veryvaluable in practice.

Nineteenth Embodiment

FIG. 41 is a cross-sectional view of the main elements of a liquidcrystal display device of the nineteenth embodiment, and FIG. 41(a) is aschematic diagram showing the alignment in the initial state when noelectric field is applied. The liquid crystal cell, which functions as abend alignment cell, has two parallel substrates 400 and 401 and aliquid crystal layer 402 filled between the substrates, constituting aso-called sandwich cell. Usually, transparent electrodes are formed onone substrate, and thin film transistors are formed on the othersubstrate.

FIG. 41(a) is a schematic diagram showing the alignment in the initialstate when no electric field is applied. The alignment in the initialstate is a homogenous alignment, which means that the molecular axes ofthe liquid crystal molecules are aligned substantially in parallel andpractically homogenous while being slightly tilted with respect to theplanes of the substrates 400 and 401. The liquid crystal molecules atthe boundaries to the substrates are tilted in opposite directions atthe upper substrate 400 and the lower substrate 401. This means that thealignment angles θ₁ and θ₂ of the liquid crystal molecules at theboundaries to the substrates (that is, the pretilt angles) have beenadjusted to have opposite signs. In the following explanations, thealignment angle and the pretilt angle represent the tilt of themolecular axis of the liquid crystal molecules with respect to a planeparallel to the substrates, and are positive in counter-clockwisedirection, taking a plane parallel to the substrates as the reference.

When an electric field that is stronger than the value in the directionperpendicular to the substrate plane is applied to the liquid crystallayer 402 in the state shown in FIG. 41(a), the alignment of the liquidcrystal changes, and the liquid crystal transitions into the alignmentshown in FIG. 41(b).

The alignment shown in FIG. 41(b) is called bend alignment. The tilt ofthe liquid crystal molecule axes with respect to the substrate planenear the substrate surfaces, that is, the absolute value of thealignment angle, is small, and the absolute value of the alignment angleof the liquid crystal molecules at the center of the liquid crystallayer 402 is large. Furthermore, the liquid crystal molecules arepractically not twisted across the entire liquid crystal layer.

Observing the transition from homogenous alignment to bend alignment indetail, it can be seen that bend alignment seeds appear in portions ofthe liquid crystal layer 402, and these seeds grow successively largerwhile eroding other regions that are still in homogenous alignment,until finally the entire liquid crystal layer is in bend alignment Inother words, for the transition of the liquid crystal layer into bendalignment, the creation of seeds, that is, the transition of microscopicregions from homogenous alignment into bend alignment is necessary.

The inventors have analyzed the transition of these microscopic regionsinto bend alignment by solving the kinetic equations of the unit vector(referred to as “director” below) for the liquid crystal moleculealignment, and have found conditions, under which the seeds can appearmore readily. This approach is explained in the following.

The alignment of the liquid crystal is expressed by the director. Thedirection n can be written as follows:

n(x)=(n _(x)(x,y,z), n _(y)(x,y,z), n_(z)(x,y,z))  (Equation 1)

As is shown in Equation 2, the free energy density f of the liquidcrystal can be expressed as a function of the director n.

f=½{k ₁₁(divn)² +k ₂₂(n×rotn)² +k ₃₃(n×rotn)²}½Δε(E·n)²  (Equation 2)

In this equation, k₁₁, k₂₂ and k₃₃ are the Frank elastic constants,representing the elastic constants for splay, twist and bend. Δε is thedifference of the dielectric constant in the direction of the liquidcrystal molecule axis and the dielectric constant in directionsperpendicular thereto, that is, the dielectric anisotropy. E is theexternal electric field.

In Equation 2, the first, second and third terms represent the elasticenergies for splay, twist and bend deformations of the liquid crystal.The fourth term represents the electrical energy due to the electricalinteraction between the external electric field and the liquid crystal.If Δε>0, then the electrical energy is minimal when n is parallel to Eand if Δε<0, then the electrical energy is minimal when n isperpendicular to E. Consequently, when applying an electric field Eabove a certain strength, the liquid crystal molecules align with theirmolecule axis in parallel to the electric field if Δε>0, andperpendicular to the electric field if Δε<0.

The total free energy F of the liquid crystal when the alignment of themolecules in the initial state is deformed by an external electric fieldcan be expressed as the volume integral over f:

F=∫f(n(x))dx  (Equation 3)

As shown in Equation 3, the total free energy F is a function defined bytaking the unknown function n(x) of the director as a variable (that is,a functional). The alignment of the liquid crystal under application ofan external electric field is expressed by the n(x) at which the totalfree energy F becomes minimal for appropriate boundary conditions. Thismeans, when the n(x) is determined at which F is minimal, then it ispossible to predict the alignment of the liquid crystal. Furthermore, ifthe time-dependent director n(x, t) can be found at which F becomesminimal for appropriate boundary conditions, then it is possible topredict the behavior of the device, such as its optical constants. Inphysical terms, this is a typical example of the principle of leastaction, and in optical terms, this is a variational minimization problemwith boundary conditions.

Equation 3 can be solved theoretically. However, it is difficult todetermine a functional form for the director n(x), because analyticmethods, such as using Euler's equation, lead to complicated non-linearequations.

In order to facilitate the solving of Equation 3, the following approachis pursued. First, the integral space is subjected to discretization bya method such as the finite element method. That is to say, the totalintegral space is partitioned into np elements, and expressed as the sumof the integrals over all elements: $\begin{matrix}{F = {{\int_{V}{{f\left( {n(x)} \right)}\quad {x}}} = {\sum\limits_{f = 0}^{{np} - 1}\quad {\int_{\Delta \quad V}{{f\left( {n(x)} \right)}\quad {x}}}}}} & \left( {{Equation}\quad 4} \right)\end{matrix}$

The following approximation is performed for the director n(x) in thepartial integral space ΔV. As shown in Equation 2, n_(x), n_(y), andn_(z) are actually functions of x, y, and z, but it is assumed that theyare constant in ΔV. Furthermore, dn_(x,j)/dx is approximated asd_(nx,j)/dx=(n_(x),_(j+1)−dn_(x,j))/Δx.

Here, n_(x,j) is the n_(x) in the j^(th) element, which is constant inΔV as explained before, but which is unknown. This approximation of n(x)in the partial integral space ΔV is coarse, but making the partitions ofthe integral space smaller can compensate this and improve theapproximation.

With this approximation, n_(x,j), n_(y,j), and n_(z,j) are constantswithin each element in Equation 4, so that it is easy to calculate theintegral itself. However, at this stage, a number of higher order termsand non-linear terms of the unknown n_(x,j), n_(y,j), and n_(z,j)proportional to the number of partitions are present in the equation forthe total free energy F, so that this equation is still difficult.However, the values n_(x,0), n_(y,0), and n_(z,0), can be easily givenas the boundary conditions.

With the above approximation, the free energy F takes on the form:

F=F(n _(xj) ,n _(yj) ,n _(zj))(0≦j≦np−1)  (Equation 5)

That is to say, the free energy F is converted from a functional withthe unknown function n(x) as a variable to a function of the unknownn_(x,j), n_(y,j), and n_(z,j). The unknown n_(x,j), n_(y,j), and n_(z,j)are the values minimizing the function F in multi-dimensional parameterspace.

As mentioned above, the bend alignment of the liquid crystal is astructure practically without twisting. As mentioned before, thedirector n is actually a function of x, y, and z, but it can beexpressed as a function of the alignment angle. In that case, thedirector n in the bend alignment can be expressed as:

n=(cos θ, 0, sin θ)  (Equation 6)

Here, θ is the tilt of the liquid crystal molecules with respect to aplane parallel to the substrates, that is, the alignment angle.Furthermore, θ is dependent only on the distance z of the liquid crystalmolecules to the substrates. FIG. 2 is a schematic diagram illustratingthese directors.

Inserting Equation 6 into Equation 4, and performing partitioning anddiscretization into np elements, the θj minimizing F is determined foreach element. This means, for each element, the θj satisfying$\begin{matrix}{\frac{\partial F}{d\quad {\theta j}} = {{\left( \frac{k_{33} - k_{11}}{d^{2}} \right)\left\{ {{\left( {\theta_{j + 1} - \theta_{j}} \right)^{2}\sin \quad 2\theta_{j}} + {\left( {\theta_{j + 1} - \theta_{j}} \right)\left( {{\cos \quad 2\theta_{j}} - \frac{k_{33} + k_{11}}{k_{33} - k_{11}}} \right)} + {\left( {\theta_{j + 1} - \theta_{j}} \right)\left( {{\cos \quad 2\theta_{j - 1}} - \frac{k_{33} + k_{11}}{k_{33} - k_{11}}} \right)} - {\frac{{{\Delta ɛ}({dE})}^{2}}{\left( {k_{33} - k_{11}} \right)}{sin2}\quad \theta_{j}}} \right\}}}} & \left( {{Equation}\quad 7} \right)\end{matrix}$

is determined. In this equation, d is L/np, wherein L is the distancebetween the substrates.

However, it is not easy to formulate and solve a system of np difficultnon-linear equations as in Equation 7. Therefore, Equation 7 is solvedwith the following circuit analogy. The kinetic equation of thedirectors is written as: $\begin{matrix}{{{\eta \frac{\partial\theta_{i}}{\partial t}} + \frac{\partial F}{d\quad {\theta j}}} = 0} & \left( {{Equation}\quad 8} \right)\end{matrix}$

wherein η is the viscosity coefficient of the liquid crystal. Thefollowing circuit analogies are applied to Equation 8:

η→C θ _(i) →Vj  (Equation 9)

Thus, Equation 8 is transformed into: $\begin{matrix}{{{C\frac{\partial V_{j}}{\partial t}} + \frac{V_{j}}{R_{j}}} = {0\quad \left( {0 \leqq j \leqq {{np} - 1}} \right)}} & \left( {{Equation}\quad 10} \right)\end{matrix}$

As shown in FIG. 3, the circuit corresponding to Equation 10 consists ofnp CR circuits. The second term in Equation 10 represents the currentflowing through the CR circuits. Here, Rj is the resistance fordischarge relaxation, and is a voltage controlled resistance,determining the current (i) flowing through the CR circuits asi=∂F(Vj)/∂Vj.

The current i (=∂F/∂Vj) settles to zero at a certain Vj. This means, Vjis determined automatically when determining the voltage at which thecurrent through the CR circuits is zero with a circuit simulator.

Thus, by transforming the kinetic equations of the directors into anequivalent circuit, it is possible to analyze the non-linear equationsystem representing the effect of liquid crystal alignment with acircuit simulator, and to determine the relation between the externalelectric field E and the alignment (that is, the alignment angles θj).

As in this approach, the non-linear equation system representing thealignment effect is converted into a circuit by electric circuit analogyand analyzed with a circuit simulator, the equivalent circuit can besimply entered into a program, and there is no need to include acalculation for solving the equation itself. Therefore, simplificationand downsizing of the program can be achieved.

Furthermore, by using this approach to calculate the change of thealignment angle θj as the external electric field E is increased, it ispossible to determine the critical electric field Ec for liquid crystaltransition as the external electric field Ec at which the alignmentangles θj change abruptly.

FIG. 44 is an example of the calculation results of this approach, andshows the temporal change of θj when the external field E is increasedwith time. The results shown in FIG. 4 have been calculated setting theboundary conditions θ₀=0.1 rad, and θ_(np−1)=0.1 rad, and withk₁₁=6×10⁻⁷ dyn, k₃₃=12×10⁻⁷ dyn and Δε=10. As shown in FIG. 4, whenstarting to apply the electric field, all alignment angles θj arecomparatively small, and the liquid crystal is in homogeneous alignment.However, after a certain time has passed, that is, when the externalelectric field E exceeds a certain value (E>Ec), the alignment angles θjsuddenly change, and the transition takes place. The absolute values ofthe alignment angles θj after the transition increase from the vicinityof the substrates to the center of the liquid crystal layer, and it canbe seen that the liquid crystal after transition is in the bendalignment.

The smaller the critical electric field Ec is, the faster the alignmentof the liquid crystal changes from homogenous alignment to bendalignment. With the approach described above, the critical electricfield Ec was calculated for various parameters determining the alignmentof the liquid crystal. As a result, it was found that the criticalelectric field Ec is susceptible in particular to the elastic constant(splay elastic constant) of the liquid crystal and to asymmetries of thepretilt angles.

FIG. 45 shows the result of determining the relation of the splayelastic constant k₁₁ and the critical electric field Ec. The resultsshown in FIG. 45 have been calculated setting the boundary conditionsθ₀=+0.1 rad, and θ_(np−1)=−0.1 rad, and with k₃₃=12×10⁻⁷ dyn and Δε=10.As shown in FIG. 5, the larger the splay elastic constant k₁₁ is, thelarger is the critical electric field Ec. In particular in the range ofk₁₁>10×10⁻⁷ dyn, Ec increases sharply with k_(11.)

Consequently, in order to achieve a swift liquid crystal transition, itis advantageous to set the splay elastic constant k₁₁ to less than10×10⁻⁷ dyn, preferably to not more than 8×10⁻⁷ dyn. There is noparticular lower limit for the splay elastic constant k₁₁, but it ispreferable if the splay elastic constant k₁₁ is at least 6×10⁻⁷ dyn,because it is usually difficult to synthesize or prepare liquid crystalmaterials with k₁₁<6×10⁻⁷ dyn.

There is no particular limitation with regard to the liquid crystalmaterials having such a splay elastic constant k₁₁, and suitableexamples include pyrimidine type liquid crystals, dioxane type liquidcrystals, and biphenyl type liquid crystals.

The asymmetry of the pretilt angles can be expressed by the difference(Δθ) of the absolute tilt angles at the upper and lower substrates.

Also, as mentioned above, the pretilt angles θ₀ and θ_(np−1) haveopposite signs, so that the difference (Δθ) of the absolute values ofthe pretilt angles can be expressed as Δθ=|θ₀+θ_(np−1)|.

Curve a in FIG. 46 shows the calculated relation between the difference(Δθ) of the absolute values of the tilt angles at the upper and lowersubstrates and the critical electric field Ec. Curve a in FIG. 6 hasbeen calculated for k₁₁=6×10⁻⁷ dyn, k₃₃=12×10⁻⁷ dyn and Δε=10. Curve ain FIG. 6 shows that the larger the difference Δθ of the pretilt anglesis, the lower is the critical electric field Ec. In particular in arange of Δθ≧0.0002 rad, Ec decreases sharply as Δθ increases.

Consequently, to achieve a swift liquid crystal transition, it isadvantageous to set the difference Δθ of the pretilt angles to at least0.0002 rad, preferably to at least 0.035 rad. Furthermore, there is notparticular limitation with regard to an upper limit for the differenceΔθ of the pretilt angles, but preferably, the difference Δθ of thepretilt angles is set to less than 1.57 rad, more preferably to not morethan 0.785 rad.

The absolute values of the pretilt angles θ₀ and θ_(np−1) are preferablyset to more than 0 rad and less than 1.57 rad, more preferably to atleast 0.017 rad and at most 0.785 rad. The pretilt angles can beadjusted by forming suitable liquid crystal alignment films on thesubstrate surfaces by such methods as oblique deposition or LangmuirBlodgett (LB) deposition. There is no particular limitation with regardto the liquid crystal alignment films, and suitable examples includepolyimide resins, polyvinyl alcohols, polystyrene resins, polycinnamateresins, chalcone-based resins, polypeptide resins, and polymer liquidcrystals. Furthermore, in addition to the selection of the material forthe liquid crystal alignment film, the pretilt angles can be controlledby adjusting such parameters as the tilt of the deposition directionwith respect to the substrate surface in case of oblique deposition, orthe lifting speed of the substrates in case of LB deposition.

The critical electric field Ec is also influenced by non-uniformities ofthe electric field in the liquid crystal layer, because bends of theelectric field in the liquid crystal layer influence the stability ofthe alignment of the liquid crystal molecules. Non-uniformities in theelectric field can be expressed by the ratio E₁/E₀ of the main electricfield E₀ applied substantially uniformly to the liquid crystal layer andthe secondary electric field E₁ that is applied non-uniformly. Here, E₁is the maximum value of the applied secondary electric field.

The relation between the non-uniformity E₁/E₀ and the critical electricfield Ec can be determined as follows, with the above-describedapproach. That is to say, the change of the alignment angles θj whenincreasing the main electric field E₀ is calculated under the conditionthat a uniform main electric field E₀ is applied as the externalelectric field E to the liquid crystal layer, and superimposed with anon-uniform secondary electric field E₁. The secondary electric field E₁increases together with the main electric field E₀, so that the E₁/E₀ isconstant at a predetermined value. From the result of the calculation,it is possible to determine the critical electric field Ec for liquidcrystal transition as the main electric field E₀ at which the alignmentangles θj change abruptly.

FIG. 47 is an example of the calculated critical electric field Ec,calculated with the above-described approach for a varying E₁/E₀. Theresults shown in FIG. 7 have been calculated setting the boundaryconditions to θ₀=+0.26 rad and θ_(np−1)=−0.25 rad, and with k₁₁=6×10⁻⁷dyn, k₃₃=12×10⁻⁷ dyn and Δε=10. As shown in FIG. 47, the larger E₁/E₀is, that is, the larger the non-uniformity of the electric field is, thelarger is the critical electric field Ec, which becomes infinitely smallnear E₁/E₀=1. It seems that this is, because when there are bends in theelectric field in the liquid crystal layer, then the homogeneousalignment is more unstable than when the electric field is uniform, andas a result, the transition to the bend alignment accelerates.

Consequently, to achieve a fast liquid crystal transition, it isadvantageous when the a spatially non-uniform electric field E, isapplied to the liquid crystal layer together with a practically uniformmain field E₀. It is particularly advantageous to set 0.01<E₁/E₀<1. Inthe range of E₁/E₀≦0.01, it is difficult to attain a satisfying effectof promoting the liquid crystal transition by application of anon-uniform electric field, and in the range of E₁/E₀≧1, the appliedvoltage becomes too large, so that there is the problem that it is notsuitable for practical use. It is preferable to set 0.5≦E₁/E₀≦1.

Using the voltage applied between the source electrode of the thin filmtransistor and the transparent electrode, the non-uniform electric fieldE₁ can be applied to the liquid crystal layer in a directionperpendicular to the substrates. It is preferable that the non-uniformelectric field E₁ is an ac electric field with a frequency of at most100 kHz, and it is preferable that its amplitude attenuates over time.

It is preferable to satisfy a combination of two or all three of thethree conditions for lowering the critical electric field Ec, namely theconditions for the splay elastic constant (k₁₁), the asymmetry of thepretilt angle (Δθ), and the non-uniformity of the electric field(E₁/E₀). Combining these conditions, the critical electric field Ec canbe lowered even more reliably than when only one of the conditions issatisfied.

For example, curve b in FIG. 46 shows the result of the calculationsunder the same conditions as in curve a of FIG. 46, except that anon-uniform electric field E₁ is applied in addition to the practicallyuniform external electric field E₀. Curve b shows the results forE₁/E₀=0.03. As can be seen by comparing curve a and curve b in FIG. 46,the critical electric field Ec is lowered further, and a swifter liquidcrystal transition can be achieved by satisfying a combination of thetwo conditions of asymmetry of the pretilt angles and electric fieldnon-uniformity.

INDUSTRIAL APPLICABILITY

With the configurations as explained above, it is possible to attain allobject of the present invention.

In accordance with the present invention as described above, with amethod for driving a liquid crystal display device using OCB cells inwhich an ac voltage superimposed with a bias voltage is continuouslyapplied to a pair of substrates, or a step of applying an ac voltagesuperimposed with a bias voltage to a pair of substrates, and a step ofapplying an open state or a low voltage state are repeated inalternation, it is possible to obtain an OCB liquid crystal displaydevice of the bend alignment type without display defects, with fastresponse times, which is suitable for moving images, which has broadviewing angles, and in which a transition from splay alignment to bendalignment can be accomplished substantially reliably and in a very shorttime.

Furthermore, the effect attained with the present invention is that itis possible to obtain a liquid crystal display device for OCB displaymode with fast response times and broad viewing angles, made of activematrix type liquid crystal cells, in which a transition from splayalignment to bend alignment can be achieved reliably and fast.

Furthermore, in accordance with the present invention, in an activematrix liquid crystal display device, including an array substrate, anopposing substrate, and a liquid crystal layer arranged between thearray substrate and the opposing substrate, wherein pretilt angles ofthe liquid crystal at an upper and at a lower boundary of liquid crystallayer have opposite signs, and in a liquid crystal cell in splayalignment, which has been subjected to a parallel alignment process, theliquid crystal is in splay alignment when no voltage is applied,wherein, before liquid crystal display driving, an initializationprocess for a transition from splay alignment to bend alignment isperformed by application of a voltage, wherein the liquid crystaldisplay driving is performed in the bend alignment attained by theinitialization, a liquid crystal display device for OCB display modewith fast response times and broad viewing angles, made of active matrixtype liquid crystal cells, in which a transition from splay alignment tobend alignment can be achieved reliably and fast, by providing eachpixel with at least one transition-inducing transversal fieldapplication portion due to which a transversal electric field isgenerated, and applying a continuous or intermittent voltage between thepixel electrode and the common electrode, creating transition seeds ineach pixel and transitioning the pixels from splay arrangement to bendarrangement.

Furthermore, in accordance with the present invention, an OCB displaymode alignment liquid crystal display element is a parallel alignmentliquid crystal display element including a liquid crystal layer disposedbetween a pair of substrates and a phase compensator arranged outsidethe substrates, achieving a reliable and fast splay—bend alignmenttransition, which is very valuable in practice.

Furthermore, in accordance with the present invention, a method ofapplying an electric field to a liquid crystal disposed between a firstsubstrate and a second substrate arranged in opposition andtransitioning the alignment of the liquid crystal into bend alignmentcan cause swift transition of the liquid crystal into bend alignment bysetting the splay elastic constant k₁₁ of the liquid crystal in therange of 10×10⁻⁷ dyn≧k₁₁≧6×10⁻⁷ dyn, and satisfying the relation 1.57rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is the absolute value of a pretiltangle of the liquid crystal with respect to the first substrate and θ₂is the absolute value of a pretilt angle of the liquid crystal withrespect to the second substrate.

Furthermore, in accordance with the present invention, a method ofapplying an electric field to a liquid crystal disposed between a firstsubstrate and a second substrate arranged in opposition andtransitioning the alignment of the liquid crystal into bend alignmentcan cause swift transition of the liquid crystal into bend alignment bysetting the splay elastic constant k₁₁ of the liquid crystal in therange of 10×10⁻⁷ dyn≧k₁₁≧6×10⁻⁷ dyn, and, when the electric field is amain electric field E₀ applied uniformly over space, to which asecondary electric field E₁ applied non-uniformly over space issuperimposed, satisfying the relation 1.0>E₁−E₀>1/100.

Furthermore, in accordance with the present invention, a method ofapplying an electric field to a liquid crystal disposed between a firstsubstrate and a second substrate arranged in opposition andtransitioning the alignment of the liquid crystal into bend alignmentcan cause swift transition of the liquid crystal into bend alignment bysatisfying the relation 1.57 rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is theabsolute value of a pretilt angle of the liquid crystal with respect tothe first substrate and θ₂ is the absolute value of a pretilt angle ofthe liquid crystal with respect to the second substrate, and, when theelectric field is a main electric field E₀ applied uniformly over space,to which a secondary electric field E₁ applied non-uniformly over spaceis superimposed, satisfying the relation 1.0>E₁−E₀>1/100.

Furthermore, in accordance with the present invention, a method ofapplying an electric field to a liquid crystal disposed between a firstsubstrate and a second substrate arranged in opposition andtransitioning the alignment of the liquid crystal into bend alignmentcan cause swift transition of the liquid crystal into bend alignment bysetting the splay elastic constant k₁₁ of the liquid crystal is in therange of 10×10⁻¹ dyn≧k₁₁≧6×10⁻⁷ dyn, satisfying the relation 1.57rad>|θ₁−θ₂|≧0.0002 rad, wherein θ₁ is the absolute value of a pretiltangle of the liquid crystal with respect to the first substrate and θ₂is the absolute value of a pretilt angle of the liquid crystal withrespect to the second substrate, and, when the electric field is a mainelectric field E₀ applied uniformly over space, to which a secondaryelectric field E₁ applied non-uniformly over space is superimposed,satisfying the relation 1.0>E₁−E₀>1/100.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The embodimentsdisclosed in this application are to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription. All changes that come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

What is claimed is:
 1. A method of driving a liquid crystal displaydevice configured to perform an initialization process for transitioningan alignment state of a liquid crystal layer disposed between a pair ofsubstrates from splay alignment to bend alignment by applying an acvoltage superimposed with a bias voltage to the substrates, beforeliquid crystal display driving, comprising: alternately repeating a stepof applying the ac voltage superimposed with the bias voltage to thesubstrates and a step of putting the substrates into an electricallyreleased state, thereby causing the liquid crystal layer to transitionto the bend alignment.
 2. The method according to claim 1, wherein theac voltage superimposed with the bias voltage has a frequency in a rangeof 0.1 Hz to 100 Hz and has a duty ratio in a range of 1:1 to 1000:1. 3.The method according to claim 1, wherein the liquid crystal displaydevice is an active matrix liquid crystal display device, and the acvoltage superimposed with the bias voltage is applied between a pixelelectrode of the active matrix liquid crystal display device that iscoupled to a switching element formed on one of the substrates and acommon electrode formed on the other substrate.
 4. The method accordingto claim 3, wherein the ac voltage superimposed with the bias voltage isapplied to the common electrode.
 5. The method according to claim 1,wherein a value of the ac voltage superimposed with the bias voltage isset to a value larger than a minimum value required for transitioningthe liquid crystal layer from the splay alignment to the bend alignment.6. A liquid crystal display device configured to perform aninitialization process for transitioning an alignment state of a liquidcrystal layer disposed between a pair of substrates from splay alignmentto bend alignment by applying an ac voltage superimposed with a biasvoltage to the substrates, before liquid crystal display driving,comprising: a voltage application means that alternately repeats a stepof applying the ac voltage superimposed with the bias voltage to thesubstrates and a step of putting the substrates into an electricallyreleased state to be in a charged and held state.
 7. The liquid crystaldisplay device according to claim 6, wherein the liquid crystal displaydevice is an active matrix liquid crystal display device having aswitching element.